it depends on what you have in mind when you say "need"!

altcoins are supposed to be alternative projects to bitcoin that try and do things that bitcoin can not do. for example bitcoins algorithm is Poof of Work. you can't just change that, so you create a new project and start developing different technologies. PoS comes in. then Proof of Space, and a couple of the Poof of ... that i do not recall right now. then you get technologies that are different, sometimes better, sometimes worse.

so do we need them? yes we need them to create new things. possibly even something better than bitcoin someday. hopefully.

but if we consider the current state of altcoins, the answer is no. we do not need 90% of them. because they just copy paste each other without any improvement or anything.

as a trader's perspective, yeah we need them. the more the merrier. we get more pump and dumps and lots more profit.