We need to make reservations about half of the time, mainly during the dreaded "Snowbird" season here in southern Arizona.  If making a reservation online as we usually do it is easy to see if the site is occupied for the day before we arrive.  If it is we will tend to arrive no earlier than the required checkout time knowing that we may have to wait a bit for the site to be serviced.
I have offered to do any required maintenance in exchange for being allowed in before the posted check-in time and often they say that is fine as most sites require little if any work.  What is a PITA is when folks overstay their reservation.  If checkout time is noon you need to be GONE by noon, not just starting to pick up the chairs, hook up your rig whatever.  You have no idea when I will be arriving and at noon that site is no longer yours.
OK, rant off... for now.
:C