In the first hour of the film Joker (2019) there are several references to an infestation of rats in the city. It's mentioned on news broadcasts several times, even being dubbed "super rats" at one point, and Murray Franklin makes a joke about them in one of his openings (something about "super cats"). But it seems like nothing is ever made of this detail later.
What was the purpose of this detail in the movie?
Gotham is an allegory for New York. Rats have always been a big problem in New York but there was a particularly bad time in the 70s and 80s where there were reports of "Super Rats" and rats attacking people (NYT). There were also a number of garbage strikes leaving the city covered in rubbish (NYT). The subway scene is also a direct reference to the vigilante subway shooting in New York around the same time period. These details link Joker to this time frame and the issues faced at the time of high tensions and violence.
It shows the lack of problem solving skills of the Government from the time Bruce was a small boy until he became Batman we see that the Trash problem continued and was never really solved, what I mean by this time many politicians came and went but never successfully solved the problem of the people which adds to the agony of people living in Gotham.
Also I believe this is one of the finer ways the movie relates to the Dark knight series, cause in the entire Dark knight series we see that the Trash problem is dominant, like Gotham is made to look like this filthy and over populated city with trash everywhere.
In addition to the already provided answers, I had another thought which I have found some like-minded validation online for. I think another purpose of the super rats was metaphorical and even foreshadowing. The rats are a metaphor for the escalating violence and corruption in the city. Possibly even a foreshadowing of the uprising of the poor. They always been overlooked before and even now are treated as a joke as seen on Murray Franklin. The rich ignore the problem and blame the rats, when they're really only getting stronger because the government won't take out the trash per se.
Here's an article with a slightly different take on the metaphor, but still similar.
We all saw them, right? Hiding in the corners of the screen, scurrying out of sight, mentioned on the news and on the radio? Super rats. Gotham city is infested with them.
I mean, that’s cool world building and all, but what does that actually mean? Well, in my view – it’s a metaphor. As the city deteriorates, with social care being cut as the rich grow richer; the rats represent what happens to things when they go unloved. Rats, on their own, don’t pose much of a threat. When they’re left – along with the garbage that’s been abandoned on the streets – they evolve into mutant rats. It’s a metaphor for the issues with the mentally ill. Gotham City is ignoring its most vulnerable people – and it will cause a problem for the city.
Additionally, this allows for a bit of an Easter Egg hunt. There are several scenes where pretty big rats are seen in the background, but are easy to miss. So additional to all of the above, also meant as a bit of an inside joke?
Source: https://www.imdb.com/title/tt7286456/trivia?item=tr4752686
I found this fact on IMDB's page
The "super-rats" segment on Murray Franklin's show and in the news references Ratcatcher, an obscure D-list villain who could mind control rats and is set to appear in James Gunn's The Suicide Squad.
Found here: https://www.imdb.com/title/tt7286456/trivia?item=tr4752686 Ctrl + F for "rats"
