I could be wrong, but I think many toilet valves (or the water supply line to the toilet) are multi turn valves and most likely is screwed onto a threaded stub out.
If you want to replace that old, hard to turn, corroded valve, etc., get a new quarter turn valve which is probably also called a ball valve..
Be careful removing the old valve from the stub out. Use 2 wrenches so you don't break off the old threaded stub out if it's sweated to the old supply line.
Freepers know everything don't we? !
Now if only I could get some advice on my broken down personal relationships,.....
You are the only common denominator in all of them. :)