Most of the reasons for instrumental breaks listed here seem to be justifications based on practical need. While these are, indeed, important I have always felt that the most successful instrumental breaks that I have heard help to move the song forward in some way. This could be by emphasizing (or playing off) the melody, introducing a new element into the melodic or rhythmic flow (change in key, possibly), or to set up the song's resolution in the following verses or choruses.

I have always enjoyed instrumental breaks that surprise you by pulling out the song's implication, rather than those that say "Look at ME! My parents paid for a lot of lessons!" Unfortunately, in my experience guitar wanking sells a h*ll of a lot more beer than subtlety.