From the StackExchange Blog:
The editing feature is there so that old question/answer pairs can get better and better. For every person who asks a question and gets an answer on Stack Overflow, hundreds or thousands of people will come read that conversation later. Even if the original asker got a decent answer and moved on, the question lives on and may continue to be useful for decades.
It is OK to edit a question to make it more general. With the power of editing comes the power to take someone’s selfish, very specific question, and edit it a little bit until they’re asking the more general question that hundreds of people encounter. For example, if someone asks, “I set up a web server at home but I can’t access it from work,” it’s OK to rewrite the question as, “What things should I check when a web server running at home is not visible on the Internet?” In fact, sometimes selfish, stupid questions of the “do my homework” variety can be easily edited into a form where the answer will provide an extremely valuable resource for the internet at large.