Partly to show that they were removed.
Otherwise, if there is damage and the plugs are in, someone could argue that they were never removed.
It would be hard to prove that they were removed and reinstalled unless you video recorded everything.
Sometimes things take time to drain and the person might not want to wait.
Partly, it's a time issue because it takes time to reinstall the plugs.
Partly, it's probably about getting the opening job.
Partly, it’s about quality control.
Before you leave the job, you can verify that all plugs are removed, especially if you have an apprentice who was responsible for making sure that everything was drained.
If the plugs are reinstalled, you will sometimes forget if you removed and reinstalled a particular plug.
Or, the customer will come out and see the plugs in and they will call and ask why the drain plugs were not removed.
So, you explain that you remove and then reinstall them and many customers will wonder if you really removed them.