What is the real purpose of hijab?

I was looking for a bus station, so I asked a girl to show me (the girl was wearing full hijab; everything is covered except her eyes). She told me "let me walk you by, I'm going there too". When we were together I noticed that she was attracting men and drawing a lot of attention, even though she was covered more than me (I wear hijab but not full; pants and a long top). I got confused. My question is: What is the real purpose of hijab? Is it her fault that she is causing fitna? Do I get sins for not wearing a full hijab even though am not causing fitna (am not a very attractive person)?