Why did they cut off my water?

Any helpful Jellies out there know about plumbing?
I had slow drains in my room, and figured that was just crappy outdated French tech. I was able to quicken up the drain in my shower with some Dran-o equivalent, but the one in the sink eventually blocked up.
So, a plumber came in and de-clogged my sink while I was gone (I have tested and it drains fine now), but I was informed that I have no running water and won’t til Tuesday.

Why was it necessary to cut off the running water to my room, if the sink has already been unclogged?! I thought running water and drains were two separate pipes…