Why I can not move created rectangle?

Doesn't help much, because you can't move the selected area or correct/change its borders after selection. That means, if you did not hit the area to be measured pixel-exactly at the first go, you have to repeat the measurement until it fits as desired sometime.

Lacks the ability to fine-tune the measurement, either with mouse, or by keyboard (control-key+arrows).
This makes for very limited usability

Not working on a lots of site (mainly wordpress site).
Lots of bugs, including select elements on page when using it.
Page with top padding brake the plugin.
Bug with high dpi screen on windows, eg 4k rez with windows upscalling to 275%.