My theory is that all the severed employees are doing various maintenance tasks to keep the floor functioning. Like the hospital in Yes, Minister that is closed to the public but has 500 employees, all of them overworked.[1]
Take O&D for instance, their full time job appears to be to just create art and handbooks used on the severed floor. Perhaps all the departments are like that. MDR could be doing some sort of ongoing maintenance for the severed system itself, like emotion or memory control of the employees on the floor.
I believe the "point" of the severed floor is not the actual work that's being done, but the act of keeping them occupied while they are experimented on. Besides Mark S and Cold Harbor, I believe there are hints of other experiments being run. The dreams Irving B has during work seem unique to him, and it's uniquely affecting his outie as well. I also believe recent events in Season 2 with Dylan G could be the start of another experiment.
^1: https://www.youtube.com/watch?v=JAk448volww