On Tuesday, January 31, 2017 at 8:01:37 PM UTC-5, DerbyDad03 wrote:

Why do you have to shut off the main valve just to change a washer?

I don't know if you mean a faucet washer or a washing machine, but it
doesn't really matter. Each fixture should have it's own shut off so it
can be shut off without impacting the entire house. If they don't, that's
not on the town, that's on the homeowner.

In theory you are correct.

But I've never seen a house where all the angle stops shut off completely. Even if 9 out of 10 work fine, the washer you need to change is guaranteed to be on sink number 10.