Flew LGA - CLE a few weeks ago and it was about 50 minutes between the time we pushed back until we took off and this was at 9 AM. However, the flight got to CLE ontime. They pad these schedules because as others have said, they have an idea on how long it will take to get from the gate to the runway and then from one airport to the next.
This just shows how messed up LGA is along with being another dumpy east coast airport. DL blocks LGA - CLE for more time than CLE - MSP (which is about 200 miles longer) because MSP doesn't have issues like LGA.