What is the removable rear cover on a 19-inch cabinet used for?

It normally closes the rear of the cabinet, but it can be removed when access to the back of the rack is required.

With the cover off, most of the rear of the internal rack becomes accessible. That can be useful when installing or servicing a freestanding or otherwise rear-accessible cabinet, and it can allow properly supported equipment to project beyond the normal enclosed depth where there is actually space behind the cabinet for it to do so.

The panel itself is essentially a cover rather than a major structural member.

There is one practical catch: decide what you want to do with it before mounting the cabinet flat against a wall. Once the wall is directly behind it, access to the retaining screws becomes more philosophical than practical.

And, more fundamentally, a wall immediately behind the cabinet leaves nowhere for equipment to project through the rear anyway.