When you pop your back the popping sound you hear when popping your back is caused by the slight separation of your joint surfaces.
The popping of your back helps to relieve the pain and helps your back feel good due to the separation of your joint surface and possible fluid buildup.
Popping your back also helps to relax your muscles as well and improve the range of motion of your joints.
It's okay to pop your back sometimes but just don't overdo the popping of your back as it could cause more problems.
But you do get temporary relief of back pain and helps your back feel good for at least a short period of time when you do pop your back.