Like, imagine you're in a sport room, and you bounce a basketball. Even though sound waves are moving at the speed of sound, it's still likely to take an amount of milliseconds for the first reflections (known as early reflections) to bounce off the walls and ceiling and get back to your ears.
One aftereffect of pre-delay is to help make the source seem like it's closer, and another is to help make the room seem larger. That will sound a bit contradictory, but it makes sense whenever you analyze it.
With pre-delay added, a source sounds closer as you hear the initial part of the sound dry prior to the reverb kicks in after the pre-delay. This will allow an oral, for instance, to cut through more and be clearer. The pre-delay also makes the reverb sound bigger due to the correlation between room size and how long it will take the reflected sound to get back to your ears.