I create side adventures under a couple of circumstances:
1) I'm not ready. I have a few one-nighters planned out in advance - just in case sort of things. If I have not prepared for the campaign main stream adventure, I'll run one of these little diddies. There is always some way of linking it to the main adventure, but nothing terribly important.
2) Players are missing. There are times when not everyone can show. More often then not, this is not a problem, but on a night were a very important part of the story occurs, I'll break the continuity and run an off game. It rewards those who show up, but doesn't heavily penalize those who cannot make the game.
There have only been a couple nights where I have had to run off games, usually my group is good about attendance and I try and plan adventures 2 weeks in advance.
Erge