The overhead clear, or any clear in general, in Badminton is used for many different reasons:
To push your opponent to the back of the court - this makes their next shot more predictable because they will not make a good smash so they will most likely clear back or drop. Depending on your opponent, their returning shot may be slightly different
To Move your opponent around and tire them out
To give yourself time to recover
For these things to work, you must hit a good clear. It should be high enough to give yourself time to recover, and far/deep enough or else your opponent will make you eat bird (smash it in your face) or put a dent in the ground.
If you don't hit a proper clear, you will have set your opponent up for a good smash - they can place it easy as well.