Well because social networking is the future of marketing (viral marketing at it’s best!)AND it is a great way to keep up with industry events! I use mine strictly for industry stuff, everything is hidden and private not only to protect myself but also to protect the industry people that I deal with 
I only add people that I know I can trust and only my friends can see what is going on in my life
You can make private groups to market your sites etc.. the benefits are up to you and how you use Facebook to your advantage There are ways to market and advertise if you search and learn Facebook.
If I wanted a personal one I could do that as well but who has the time..lol