PMing is an odd thing. Sometimes it's difficult to figure out what you did all week, especially if you don't want to use words like 'unblocked', 'enabled' and 'drove'. You might spend a large portion of your time simply following up with other people to get them to respond to your questions (get used to opening mail from your sent items folder, replying all and saying “Any word on this?“, especially if you're waiting to hear back from another PM), and then either write those answers down or give them to the people who were waiting for them. Those people, i.e. dev and test, are way too critical path to have them micro-manage getting answers to those questions. The PM does a lot of the grunt work for the dev/test/PM team, as it should be.
It's not all grunt work, of course, but one of the yardsticks against which a PM can be measured is the success of his/her individual devs/testers. “Did they have the information that they needed -- when they needed it -- in order to do their jobs?“ Yes? Then the PM probably played a role.
There are various strategies I have picked up over the years that have aided me in this role. One of the most effective is this little phrase:
“Hi, I still haven't heard back from you on this... I really need this information by friday, so if I don't hear back from you I'm going to assume that the answer is yes.”
It's amazing what a difference it makes.