The reason I, personally, used Wikipedia the one time I referred to it is pretty simple: it's free. While a lot of other online encyclopedias of my experience are partially free, I was pretty uncertain about whether or not the specific information I required at the time would be.
"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"
"You can't erase icing."
"I can't believe it doesn't work! I found it on the internet, man!"