A range gives you flexibility. I hope for the high end, sure, but I know they'll go for the low end so I throw a range where I'd be happy with the low.
Since it's a range, it's easy to say No way I'd work for you for less than [low end].
If you give a fixed value they'll low ball it and you'll generally end up in.between theirs and yours, so you'll have to aim high.