Bajoleth's given you great info there - that's what the good Dr Johnson says they should be for!
Though I have to admit that I'm slowly formulating a theory about Intermediate Days. I'm not sure they actually help many of us, particularly if you try to use them before a DD. It's my personal belief that you need to go into a DD well-fuelled, i.e., eat the majority of your UD calories (or more), otherwise you put yourself more at risk of blowing the DD. I'm starting to wonder whether, when you need to change your rotation, it might be better to have two UDs rather than try to make one of the days an Intermediate Day. I've noticed that a number of us find that the scales get a bit sticky when we introduce too many Intermediate Days, maybe because our bodies are thinking, "Okay, what's this? There wasn't much food yesterday and there doesn't seem to be a whole lot more today..." and subsequently dial down our metabolisms. Whereas if you follow two UDs with a DD, our bodies are still expecting UD levels of calories so they happily burn off the DD calories we feed them and don't realise they're not getting more until it's time for the next UD.
Just a theory.

A theory I'll openly admit I've adopted cos I'm actually rubbish at sticking to Intermediate Days - once I've opened the eating hatch, there's going to be food going in and I find it hard to stop at the 1250 calories mark (let's face it, if I could stick to eating 1250 cals every day, I wouldn't need to do JUDDD!

).