Such an interesting question, as usual.
I agree with almost everything others have replied. What often motivates me is that I really just like to know things and very much do not like finding out that HAD I known some fact I would have not gotten into trouble, or not missed out on a great thing to see or do, or just made different choices. Just this morning I am seriously annoyed to find out that years ago I was in a town in Italy for one night, just as a convenient place to sleep, and now I learn that the church in that town contains exactly the kind of 14th century frescoes I go to Italy to see. Grrrrrrr!
I don't in general like to help people --- I'm married to someone who goes way out of his way all the time to help people and I am not generous like that --- it's really in this specific way that I enjoy giving out information. It's easy and satisfying. My year-round volunteer job is almost 100% about researching and then giving out information, and for one week every year I am literally sitting at our information desk answering questions and solving problems. It's somehow important to me, sitting at that info desk, that the 1,000th time someone asks me where the bathroom is or puts their question in a really "stupid" or overly general way, I give them a good, patient answer, instead of saying "Look at the map" or making them feel stupid for asking. I try to be like that on this forum.
While planning travel or while actually traveling, I have been (and will be) that person who needs to know something and maybe doesn't know who to ask or exactly how to ask. So, when I know something that might be helpful to someone, I feel like sharing it.