The valve you can't turn is no valve at all
A lot of older homes have shutoffs that haven't moved in 30 years — frozen open, corroded, useless. When a supply line lets go, you can't close them, so you're running for the main and shutting down the whole house. A working shutoff at each fixture means you close one valve and keep water everywhere else.
Where to add them
Every sink, toilet, and appliance should have its own stops — hot and cold. If yours are frozen or missing, adding them is a short job that pays for itself the first time something leaks. While I'm doing any alteration, I replace the old stuck ones in the same area, because they're the same age as the one that's about to fail.
Call and tell me what you've got
Walk around and try your shutoffs — gently. If any won't turn, or you don't have them at all, call. Adding working stops is one of the cheapest upgrades that genuinely protects your home.
Have a plumbing issue right now?
Call and leave a voicemail — Bob calls back during business hours. For the fastest reply (usually within the hour), email him directly.