In the TV series Bosch, the Koreatown Killer ("KTK") is bicycle-riding serial killer who chooses his victims apparently randomly, usually also taking their cell phones.
He first appears in S3E1, appearing sporadically and usually very briefly. His last appearance is
in the middle of S4, when he meets his death in a traffic accident that seems to be completely unconnected with his killings.
The police investigation gets very little attention from the show, but
Crate & Barrel do connect the dots -- after his death -- and determine that this dead guy used to be KTK.
I don't understand why this subplot exists. We never learn much about KTK, or why he kills. We spend very little time with police who are trying to catch him. Perhaps I'm forgetting something, but I don't think the show uses KTK's brief vignettes as opportunities to comment on other events that take up the show's main focus.
KTK comes and goes without any ceremony, and has really no impact on the characters and events at the center of the show.
What is the purpose of the KTK subplot? Why did they film it and give it screen time? They carry it on long enough that it seems like a deliberate choice.