Knowing that you're more likely to have a closer relationship with the grandchildren she gives you - a lot of women tend to go to their OWN family for that sort of thing, rather than their in-laws... so you've kinda got that on your side, so long as your relationship with your daughter is good.
You can also make a friend in your daughter as she grows - I would love to spend time at the movies with my mother, and being girls, we generally like the same films and stuff, so it gives me someone to watch girly flicks with.
They're more likely to have partners you like (so long as they're hetero). I think it's easier to be wooed by your child's male partner (when you're a hetero female), than it is to be wooed by your child's female partner. I think I'd probably feel less, "That b*tch isn't good enough for my child!" than I would if she was a boy with a girlfriend.