Laziness. It's usually faster to ask someone a question than to look up the answer. I used to do this on IRC when learning a new programming language and it saved me a lot of time. If you want an even faster answer, you guess the wrong answer and wait for someone to correct you (https://meta.wikimedia.org/wiki/Cunningham%27s_Law).
Sure, you might get the answer that X is the best way to do what you're trying to do, but if you read the docs, you might learn that what you're trying to do with X is already implemented and you can skip all that and just use Y. ( you not meaning you specifically here, just in general. )
:(
My common answer is "If you're too lazy to read the documentation, I'm too lazy to cut & paste it into the email".
Followed by them complaining about how I'm an asshole...
Eventually I LMGTFY. He stopped asking.
On my school's forum, people routinely ask questions which a simple Google search would have answered.
Yet typing out a question and posting it to a forum is a lot more work than searching on Google. I'd think the lazy response would be to stick to Google.