Apparently because it breaks the mental association between being in bed and sleeping, which can exacerbate sleep problems/insomnia and reduce quality of sleep.
And what about reading a few minutes to clear your mind of the previous day?
IMO unless you have trouble falling asleep, the "bed is for sleep or sex" advice is not as useful as "just make sure you get enough sleep."
I agree, just wanted to see what he would come up with.
That's begging the question (in the original and still proper use =) – the OP's claim has become a bit of a platitude on the internet and yet I've seen scant evidence for it. It may well be there, but, admittedly, as I said above, it doesn't seem to be really all that wise.