Good morning Canoe12 and welcome to the Oliver Forums. The Oliver electric jacks do extend far enough to not use any blocks at all. It's just with the use of a tall block we have discovered several benefits:
1 - Time saved in running the jacks as you don't have to run them all the way down (and back up)
2 - Energy saved in not running the jacks so much (they are hogs)
3 - Less chance of accidentally driving off with them fully extended and bending the jack (you will simply drive off the block)