In all but small companies, I think devs have little to no contact with the other product/project stakeholders. For those devs who would like to be in a position to influence/interact with the stakeholders, being a PM would eliminate some of the constraints/WTFs devs often feel. Well, not always eliminate necessarily, but minimize them, or at least provide some transparency into the part(s) of decision-making process(es) outside the devs' sphere of influence.