(A slightly belated reply because I've had erratic net access.) The feedback is a big part of it for a lot of people. We need to know that what we wrote has had an effect on people -- otherwise why put it out there rather than just printing it out and sticking it in a drawer?

There are a lot of pro authors who write or wrote fanfic (and some pro editors as well). More than the detractors realise, but one can't name names in public without being absolutely certain they're okay with it, because it could cause trouble for those writers. One or two will stand up in public and say they're proud to be a fanfic writer -- e.g. seawasp in this thread, who is Ryk Spoor.