Its just a test to dial your printer in. I shimmed my bed with 0.1mm washers. I haven’t done a full square of plastic, but I printed my first layer / z-offset print of choice in all 4 corners and center in order to verify the bed level results in octoprint were accurate.

Before hand 70% of my bed printed perfect, but one spot was a little lower, and the mesh bed leveling wasn’t accounting enough for it. Parts printed on textured sheets would not pick up the texture as well in that one spot. I like the textured look for top surfaces of control panels and such, so having an area on the bed that wouldn’t apply the texture was a bit annoying.

Tests like what you are talking about is an extreme way to verify that everything is square, or at least well accounted for in the firmware.

Also, since this wasn’t something achievable out of the box until recently, printer manufacturers are showing it off as a point of pride / as a sales tactic.

Yeahhhhh, they have a sale going on right now, but its still like 1800. I really do think that its more of a business target with that price though. While the original stick einstar won’t pick up on super fine details, a scan from it + some caliper measurements has done me pretty well with hobby projects. The price point is definitely more reasonable on that for hobbiest makers