Why was the intermission put in Once Upon A Time In America, the 3h49m version? Was it an artistic choice, to make a break in the movie, or was it placed there to make a break in the cinemas for the viewers? Or was there some rule in Hollywood that forced a break in the movie if it was so long?
In my opinion there was an intermission in Once Upon a Time in America (1984) because there was a traditional practice of having an intermission when showing long and/or high prestige movies. An ordinary movie might be short enough to show without an intermission, but a spectacular, or epic or high prestige movie was often longer than a typical movie.
And thus a break during the showing of a long and/or high prestige film could be used for audience members using the bathroom or buying popcorn and other snacks and getting some exercise after sitting for a long while and possibly going to the lobby or someplace to smoke, etc., etc.
Wikipedia describes Once Upon a Time in America (1984) as "...a 1984 epic crime drama film..."
Wikipedia also gives three separate lengths for the film. The shortest was the re-edit at 139 minutes, longer than the usual 90 to 120 minutes of typical US films in that era. The theatrical version was 229 minutes or 3 hours and 49 minutes, and the Director's cut 250 minutes (four hours and ten minutes).
The version that Sergio Leone supervised the editing of was the international theatrical version 229 minutes long. I also note that version is considered to be a classic movie.
. This condensed version was a critical and financial disaster, and many American critics who knew of Leone's original cut attacked the short version. Some critics compared shortening the film to shortening Richard Wagner's operas, saying that works of art that are meant to be long should be given the respect they deserve. Roger Ebert wrote in his 1984 review that the uncut version was "an epic poem of violence and greed" but described the American theatrical version as a "travesty". Ebert's television film critic partner Gene Siskel considered the uncut version to be the best film of 1984 and the shortened, linear studio version to be the worst film of 1984.
Considering how much Leone's version is praised by critics, Leone probably considered his version to be a work of cinematic art and a prestige film. So if Leone supervised the editing of a film that was epic in length, and which he intended to be a high prestige and high quality film, it would be natural for him to include an intermission since the film qualified on two separate counts for having an intermission.
I don't know which version(s) TK-421 saw, and whether TK-421 watched in theaters, or at home on commercial television, or at home on some type of home video, so I don't know what version(s) of Once Upon a Time in America (1984) have an intermission.
I know if I was watching any version on commercial television with regular breaks for advertisements, I would not mind the length of even the longest version so much, because I could take a break during commercial breaks.
I also know that if I was watching even the shortest version of Once Upon a Time in America (1984) in a theater or on a videotape or DVD or Blue Ray, I would appreciate an intermission. Even at home, where I could stop a home video at any point if I wanted to, and start it again later, I would appreciate an intermission as a point to do so selected by the creators of the film as the best time to pause the film and take a break without interrupting it too badly.
I note that an article says that movie intermissions are still used all over the world, and not merely in India: "Meanwhile, mid-movie intervals are alive and well overseas, from Iceland to Turkey to India."
Iceland and Turkey are both more or less European countries, so I deduce from that statement that some longer European movies still had intermissions in 2018 when that article was written.
If it was still common to have intermissions in longer European movies in 2018, it would have been still common to have intermissions in longer European movies in the early 1980s when Once Upon a Time in America (1984) was being made (it was filmed from June 1982 to April 1983). Sergio Leone was an Italian born European movie director who expected intermissions in long movies. The longest versions of the film were Leone's work, while the shortest version was cut by the American release company.
By the end of filming, Leone had eight to ten hours worth of footage. With his editor, Nino Baragli, Leone trimmed this to almost six hours, and he originally wanted to release the film in two parts, each three hours. The producers refused, partly because of the commercial and critical failure of Bernardo Bertolucci's two-part 1900, and Leone was forced to further shorten it.
So if Leone originally planned to make two movies each three hours long, it would be natural for him to put an intermission where he had originally planned to break between the two movies, though possibly he might have moved it forwards or backwards depending on which half was cut in length more.
So it seems to me that asking why there is an intermission in Once Upon a Time in America (1984) is sort of like asking why Sergio Leone wasn't cruel enough to inflict an over three hour movie without an intermission upon unsuspecting movie audiences.
Intermissions in early films had a practical purpose: they were needed to facilitate the changing of reels. When Les Amours de la reine Élisabeth (The Loves of Queen Elizabeth), starring Sarah Bernhardt, opened on July 12, 1912, in the Lyceum Theatre in New York City, the four reel film was shown in four acts, with an intermission at each reel change.
The technology improved, but as movies became progressively longer, the intermission fulfilled other needs. It gave the audience a breather, and provided the theatre management an opportunity to entice patrons to its profitable concession stand. A well-known 1957 animated musical snipe suggested, before the main feature in theatres and during intermission at drive-ins, "let's all go to the lobby to get ourselves a snack". During the 3D film trend of the early 1950s, intermissions were a necessity because even though many theaters used two projectors that could skip intermission by shifting from one reel to the other, 3D films required the use of both projectors – one for each stereoscopic image – and so needed an intermission to change the reels on both projectors.
The built-in intermission has been phased out of Hollywood films; the victim of the demand to pack in more screenings, advances in projector technology which make reel switches either unnoticeable or non-existent (such as digital projection, in which reels do not exist).
Here is a link to a discussion of movie intermissions, advocating their return.
It seems to claim that Ghandi (1982) was the last US movie with an intermission. Once Upon a Time in America (1984) is an Italian-American co production so maybe it doesn't count as an American movie in that article.
Here is a link to another article advocating the return of movie intermissions: