It’s ergonomic, doesn’t cut off circulation in your legs and provides back support that can be used even if you’re pedaling. It’s really sturdy, doesn’t make any noise and is adjustable in height. I use it with my semi-grand and concert grand. The seat on mine detaches from the leg support.
When I purchased my chair the back rest was not available so I had to buy it as an attachment. It’s not necessary, but it really does make playing and practicing more comfortable and with better posture.
The seat does swivel, but can be tightened down. I haven’t tried locking it down since I don’t mind a bit of swivel.