Sometimes the stud can also be removed

All depends on the wall structure. If not loadbearing then it comes down to wall stiffness for supporting the remaining drywall. Hence the plywood (or wall sheathing board) layer which can provide strength and support for the now much thinner wall.

In oddball wall shapes and corners sometimes the construction guys just keep adding studs until they get enough edges to attach all the drywall pieces. All that extra vertical wood can take up a lot of useful space. Rethinking the construction details can recover some useful/usable space.