What can a project manager do? That developer can't.

Make things unnecessary difficult?

Put up with the client.

Get the bonus on devs overtime?

Well, manage and coordinate the actions of every person in a project, deal with clients, act as a liaison between the team and the company, make important decisions, act in emergencies, etc.

A good PM is worth their weight in gold.

What developer don't want to do**

Protect devs from clients.
Make coffee.
Deal with other managers.

@ItsNotMyFault hmm dont know a manager that does that

I'm pushing nasty shit off to my PM that I don't want to deal with. He has to make telecons explaining to the client why we will not do XYZ, mostly because it's not in the contract, schedule or agreed payment, and that we can do this in another iteration.

Or if the client doesn't deliver his share on time, like specs, agreeing to shit and whatnot, then the PM has to make telecons and pressure the client.