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.