I think their role is to show the world that cryptocurrency exists and because of that some people think that a random altcoins that they heard from someone is the legit coins although it gather a chance to let them know Bitcoin or perhaps they heard/saw Bitcoin on TV/News. Perhaps some of these altcoins are made just to copy Bitcoin so that they could create their own monopoly and controlled coins such as fiat that they can exploit anytime they want but due to the conflict of being a centralized currency, most people now choose Bitcoin because it's decentralized that no single entities own it. And most of the time altcoins are just being used for pump and dump to make money out from investors while the others it's just shit coins with no other purpose.
There is no doubt in the fact that altcoins have helped in the publicity of the cryptocurrency industry and their various hypes have helped to bring cryptocurrency into mainstream awareness, even though we can't deny the fact that, altcoins have also influenced the negative perception about the true intent of cryptocurrencies like bitcoin which provides better alternatives to fiat and as a good asset to hold.
And also some of the altcoins have provided some specific services that contribute to the scalability of the cryptocurrency industry and make the transaction easier faster and more flexible.