This type of back is often used on sports bras as it quite secure. The straps won’t fall down and you have a wide range of movement through your shoulders. The other use is to wear with tops that have a racerback so the straps aren’t visible.
The shape in the back can make a normal fastening impossible. For this reason they often do up at the front or are elasticated and go on over your head like a camisole. You can get bras that do up normally in the back as well, they have a hook that gives the option to attach the straps to give them the racerback (see image below).