Grace periods allow you to stop visitors from being able to make bookings minutes or hours before the desired appointment slot or event.
Grace periods are useful in a number of situations. It could be that you want to have a heads up of all bookings to give you time to prepare or get to the venue if you are not there all the time.
Other uses include:
Not allowing bookings on the spot
When building managers require lists of bookings a period before they arrive.
Before events where you don't want last minute bookings.