This is an old favorite of mine by Michael Lopp (aka Rands):

Entropy Crushers

As the title of the post suggests, it is one of the best (and most succinct) descriptions for the true role of a project manager:

“They are chaos destroying machines, and each new person you bring onto your team, each dependency you create, adds hard to measure entropy to your team. A good project manager thrives on measuring, controlling, and crushing entropy.”

The post specifically targets the people usually most resistant to the  introduction of a project manager to their team: engineers and engineering managers. Rands short answer for why they should be supportive is: “do you or do you not want to be an engineer?” For the long answer – read his post.