On my ride in this morning, I was struck by how few cyclists seem to know what's going on behind them. On a personal note it's irritating because fellow cyclists might not know that you're trying to overtake - and they're taking up half the road because of their position.
On a safety note, it's important to know how many monstrous trucks or impatient vans are revving up behind you, so you can be prepared to take the necessary action.
Cars, motorbikes, vans and trucks find it reassuring to know that you the cyclist are aware of their presence behind you.
And I've seen it argued that if a car or truck can see your face occasionally, they recognise you as a human being not just a bicycle, and may behave better.
Don't take this to extremes. Looking forwards is important too. I'm talking occasional but regular glances back, not riding with your head pointing the wrong way ...
Rather pompous sounding article but good tips on "Vehicular Cycling" at Wikipedia
There's a fear-inducing American site called Bicycle Safe, worth a read
Very long but decent article about bike safety by Myra VanInwegen