Leveling

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I have a question about leveling. I have read several threads about it and do not understand the requirement for blocks or bal levelers or whatever for under the tires that many have posted. My new to me 2005 2619 owner's manual says this:
"The simplest way to level your Trailmanor is using the two-point method. After opening you trailer, place a bubble level on the refrigerator. Using the tongue jack, level the trailer front to back. Use the rear scissor jack on the low side of the trailer to level the trailer side-to-side. To lower the scissor jacks, crank clockwise with the jack handle provided in the trunk. Once the trailer is level, you can mount a bubble level permanently on the tongue of your Trailmanor to aid in future leveling."
This is all I do for leveling except I use a power drill to raise and lower scissors, and then ensure all scissors are providing support. Why does anyone use the tire levelers?