This is useful for checking whether your display uses sequential scan refresh or global refresh. Sequential scan refresh will show a tilting effect on a vertical moving line. Sequential refresh is usually better for lower input lag since the display can refresh in sync with video cable transmission (DVI, VGA, HDMI, DP) but may have more visible scan-skew artifact at lower refresh rates (e.g. 50Hz or 60Hz). Some displays with certain types of multi-scanning will create a saw-tooth artifact. Line may tilt as little as one degree. Easier to see scan skew at 60 Hz than 120 Hz. Also easier to see scan skew if viewing at a distance of at least 2 screen widths away.
If you are testing an iPad tablet screen, try both screen rotations. Some iPads use landscape scan, and other iPads use portrait scan.
If you are testing a gaming monitor, try both with and without your Blur Reduction mode (e.g. LightBoost, ULMB, DyAc, etc)
Technologies with scan-out skewing: CRT, OLED, LCD (when flicker-free/non-strobe)
Technologies without scan-out skewing: Plasma, DLP, LightBoost, ULMB
Technologies with sawtooth artifact: Some billboard displays