[i_don_t_know]

I'm not working in this area.

As others have mentioned, you don't always have a complete and accurate set of constraints. So you generate a schedule based on what you have. You hand it out to your employees. And they point out conflicts that they forgot to tell you about or that come up on short notice. So you have to rework the schedule to take them into account manually.

You could regenerate the schedule with the additional constraints. But the new schedule is likely quite different from what you originally handed out. People will have assumed the original schedule is final and they will have made plans. You cannot simply replace the schedule.

You might also already be halfway into the schedule period and the new schedule might only be fair if you had applied it from the beginning of the period.

You might be able to account for all of that in your constraints but it's going to get really hairy.