Why do we HAVE to have differing political parties?

I understand checks and balances, but why do we have to have differing political parties to ensure that?
It seems so self defeating at times. If you’re a Republican you’re automatically against anything the Dems want to do, and visa versa. It doesn’t matter who’s right or who’s wrong or what the best course of action would be for the citizens. The only purpose is to be in opposition of them no matter what.

What’s the point?