Great for keeping the body mobile and for strong core and glutes. With core strength the back is supported and the knees will have less issues with tracking or injuries.
All the time keeping the hips stable, core engaged and spine in neutral-not flat to the reformer.
The important bit is is to try and keep the back straight, tummy muscles engaged and the plie movements as smooth as possible. Its more difficult than it looks.
It is much harder than it looks as it also requires the reformer to be held out and stay in place and still whilst bending over and twisting
Being able to isolate muscle groups and make each lot work better takes lots of practice.