It's mainly got to do with converging lines. Particularly in architecture and product photography when you point the camera up/ down. By KEEPING the back parallel with these lines, you maintain them in parallel. Depending on what you're shooting, and how big the usable IC of the lens you have mounted, you can ease/lower either the front and/or rear standard(s) to change perspective and KEEP things aligned.
Grid lines help, as does a Torpedo level to ensure the rear is vertical.
Also, as I mentioned in an earlier post, keeping the back parallel to the horizon (as best as possible, which, in reality is keeping the camera back perpendicular to the horizon, albeit slightly curved) will minimise horizontal distortion (excessive "bending" of the horizon).
Lachlan.
You miss 100% of the shots you never take. -- Wayne Gretzky
Maybe I'm misunderstanding your question, but I don't recall anyone saying the rear standard must be parallel to anything. But some subjects have an apparent geometry that happens to line up nicely on plumb and level ground glass. Personally, I don't break out the calibration equipment when taking a photo of a tree.
For whatever it's worth, my post was only a response to Taija71A, who appeared to misunderstand the scope of Lachlan's post.
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