Connecting with family and friends all over the world.
Maybe you don't have family in other countries, but it really helps to aggregate all the info in one place and with the photos.
If you lock your profile down to friends only and don't enable any third party apps, then there is no problem.
You would be surprised how many people have gotten themselves into personal, criminal and financial trouble by putting too much info on Facebook. IMO this the nasty product of the decade. Rather sign your dog and cat up if you must