I've just asked my 19-year-old about your question, based on their experience with sleepovers, and here was the answer:
1) Sleepovers are generally harmless silly fun. You stay up late, share some secrets, eat some junk food, maybe watch a movie, fall asleep at about 2am.
2) Your daughter will be an adult in a few years, living away from you in a college dorm or an apartment. Its good for both kids and parents to have occasional nights away from each other during the teen years; otherwise, your daughter's first experience away from home will be that first night after she's moved to a completely different city.
3) The mischief that 14-year-olds get into tends to be much milder than the mischief that 20-year-olds get into. Learning how to pick good friends is an important life skill. If she's chosen some friends who make mildly bad choices now, she'll learn from that how to make better friends, so that by the time she's 20, she'll know how to pick friends who don't get into adult-level bad trouble.
4) If you show your daughter that you trust her, letting her pick her own friends and many of her own activities, then you're keeping lines of communication open, and she knows she has your support. If something goes wrong then, in her friendships, she'll know that she can talk to you about it; she won't feel like she has to hide from you.