It's inevitable. At some point during your weather briefing, some underpaid, underappreciated briefer will say "Pilot reports are requested." and you'll say "Certainly, my good sir or madam! Why, the only thing that gets me out of bed in the morning is the chance to help my fellow aviators!" and then you won't make one.
But let's say that you do make a non-urgent pilot report. Briefers will give that information to pilots, but what about the meteorologists who actually make the forecasts? Supposedly, they're interested in the information as well...
"NBAA is excited about the new electronic submission form that should streamline the PIREP process, and allow the information to get to the FAA air traffic controllers and the NWS Aviation Weather Center (AWC) forecasters sooner," noted John Kosak, program manager, weather, NBAA Air Traffic Services. 
but what are they used for once they're submitted?