Saying it the way you really want is often considered blunt and rude. Wrapping your message up in a sophisticated methaphore not only leaves the impression that you're well-read and civilized, it also allows you to communicate just about anything to them you otherwise couldn't say. When practised regularly it even comes in handy when they want your comment on something and you don't have a clue what they're talking about. In order not to look stupid they'll let you get away with text like this:

"The hawk and the ant both need air and air is for free..."

or this:

"An intersection is a place where two lines cross..."

(Jacco Mokveld)