It might look serviceable, but the profile of the teeth may have worn. Whilst they might look a reasonable match for the old belt (if the belt is still intact) they will not correctly mesh with the profile of the new belt.
This video shows the comparison of a worn and new pulley when a new belt is offered on.
A tight fit of the pulley onto the splines of the camshaft is also imperative.
Hopefully this is helpful!
Watch the video below: