You are the “do-er of last resort”. If something has to get done ensure the product is successful, and if it’s not naturally part of someone else’s job (or maybe if it is, but they’re too busy or otherwise not doing it!) then more often than not it falls to the PM to get it done. This is especially true in small companies who don’t have an army of specialists you can lean on.

Need to write a press release or product documentation but don’t have a PR person or technical writer? PM does it.

Need to get straight answers from the tech support team of a difficult partner that’s driving your dev team crazy? PM does it.

Need to chase down a sample unit of some cool hardware product that you might want to integrate with in your next release? PM does it.

Need to find 5 different options for HTML grid controls that can display tens of thousands of rows without crashing your browser? PM does it.

Need to verify that a big new feature doesn’t have any big gaping problems, after the QA team has already gone over it and given it the thumbs up? PM does it.

Need to figure out the first draft of the pricing model for a new product that Sales doesn’t understand how to sell yet? PM does it.

CEO was scheduled to give some random conference presentation but at the last minute can’t make it? PM does it.

I picked the list above from stuff that I or my team has done in the few months. If I thought for a while I could probably add 20 more. In theory, all of them might have been someone else’s job, but in reality if it needs to get done, the PM does it, because PMs tend to be generalists with skills across a wide variety of domains and, typically, a high level of patience for doing stuff that moves the ball forward.