I will do this all the time for a couple reasons, but the two that come up the most are...
1. I look over it and realize I'm going to end up insulting someone, this usually happens whenever I think about posting in an argument or debate or complain about anything.
2. I get way off topic. Most recent example was I was going to post in one of the threads on here about game stores mistreating people, but after I started writing I started to branch off and complain about all the people buying all the newest stuff like majoras mask 3d and amiibos for a profit by making a monopoly on it. So I just deleted it all since I had 2 sentences about game stores and 2 paragraphs about guys on craigslist with 13 of each amiibo and 4 majoras mask 3ds consoles :p
Good thing you deleted it! We don't tolerate wild tangents here!