> It doesn't matter if there's a written contract... if you are working on a venture together with some sort of verbal understanding, in California (I'm assuming in the rest of the USA too) that is just as legally binding as a written contract

That is true, but only if you can prove there was an oral agreement. That is why written agreements are considered better, you have physical proof.

With an oral agreement you'll need to convince a judge or jury that both sides agreed to said contract, otherwise you could be SOL.