First off, buy it used at $100-115... You won't pay $260.
Secondly, it is the ease of use. Very high contrast and easy to spot the DOF at given distances. The whole system is easy and accurate. You get the sensor plane equal with the target mainly for consistant results. The build is excellent, with integrated magnets to lock the ruler in at fixed degrees. Can be mounted on a tripod for easier calibration.
Sure, you can do it other ways... But this is a TOOL that makes it EASIER.
You don't have to use a hammer to drive a nail, but if you do, the job is easier and takes less time. Same with Lens Align.
I did the same as you printing a target, slapping it on a flat surface and shooting until I got the sharpest result. My result? All but one were off compared to Lens Align...
My 135L I did at +9, stayed at +9.
135L with 1.4x TC was at +9, now at +7.
300 f/4 was at +4, now at +1.
300 + 1.4TC was at +4, now at +2.
Sigma 30 was at 0, still at 0.
Still yet to do my 24-105 and 12-24 (possible really only at the long end).
Conclusion is, I spent a LONG time calibrating those lenses to a nice, detailed target to compare sharpest image. And yet, Lens Align showed my flaws with all but one.
You don't need this tool to microadjust, it just is more accurate and easy to use IMO.