Name one.
You're late:
Just off the top of my head:
1) It's popular. Just go to the women's section of pretty much any department/retail store. Unless it's Fall/Winter attire a lot of the clothing for women feature cuts that show some cleavage. It's more socially acceptable for women to show cleavage for a multitude of reasons, and because of that it's popular. For that reason you can have a lot of women buying low-cut tops and never once thinking it's for a man's benefit. It's just what they see other women wearing.
2) It's comfortable.
3) It cooling in the summertime.
4) They like it. And whether or not a woman likes something often has no basis in what the men around her like.
And those are really just the first few reasons that immediately jumped out in my mind. If someone went and tapped GirlGAF for their collective opinion, I'm sure we'd get even more.
What's pretty strange to me is that there are apparently guys who really think that the only reason a girl would wear a low-cut top is to display her boobs to men. That's crazy to me.