For those who care about the technical details (all other can ignore my post):
It is not really a different skin. In fact, they are completely different chat programs agreeing to the same chat protocol standard called “Matrix”. And each developer team has different goals they want to achieve. Some focus on multi-account support and text only, others want to support the full specs and so want to be a full alternative, and some just want to be as light weight as possible to get integrated into Linux phone SMS program.
As you see there is a good reason to have different clients. Beside of the use-case, the main benefit however is, that they are completely independent of Element (the official chat program) and so if Element becomes bad, there are plenty of alternatives that probably make it better (whatever you call “better” is up to you). That would not happen on “just skins”.