Because you cannot drive after being on duty for 14 hours.
You can get a 2 hour extension once a week, but that is reserved for "unexpected" circumstances. You should not get a DOT extension just because "Bob" needs a little help.
you have to have 10 hrs off.
You need 10 hours off to reset your 14. You can work, say 10 hours, clock out, and clock back in.
You now have 4 hours available left to work. You do not need 10 hours off to clock back in. But you only have 4 hours available.
The issue appears when a driver works 11 hours and then starts the next morning without having 10 hours off. He only has 3 hours available, not enough to do his package route.
You cannot drive after being on duty for 14 hours without taking 10 consecutive hours off. Your on duty time does not have to be consecutive. You add up all your on duty time you had before a consecutive 10 hour off duty.