The post trip allows you to identify things that need repair before you are getting ready to pull out and hit the road at the start of your shift the next day.
It gives you time to get the soft tire or gouged steer or leaking air bag or burned out light, etc., etc. repaired during your down time so you can get moving at the start of your next shift, not three hours later.
The other day I caught a flat tire on my rig during my Post-Trip. I hadn't noticed anything, it may have gone flat on my empty run back to the barn, because I think I would have noticed something when I was full.
It was fixed while I was off duty and I wasn't standing around at 0430 the next morning trying to figure out how to get it fixed or get another trailer so I could make my pick up time.
It's really a matter of time management. Catch what needs to be fixed when there is time to get it fixed without screwing up your schedule.