One important reason is that you can't be good at everything. Having someone else who can focus on skills A & B while you focus on skills C & D is nice, if you can trust the other person to be competent. Especially consider that skills A & B might be things you hate, eg paying taxes and doing bookkeeping.

Another big reason is that if I put a lot of work into a new design or a feature, I'm in the worst position to critique it. You need someone else who can provide objective feedback. Customers can do this to some extent, but it is very easy to dismiss their suggestions.

Related to that 2nd point, it is often tempting to say, "this is good enough" when you have to do all the work yourself. Having someone to push you to do better can really improve the quality of your output.