Chain has to be checked in several different places, because it wears out differently along its length. So, you will have various readings for sure. Also, it would be much more precise if this device could measure chain links more apart say 150-200mm
It's worth jack sh*t because it measures from the left side of the roller to the right side of the roller, meaning that it not only measures chain stretch, but also roller wear, roller play and roller diameter (which is slightly different for every chain manufacturer). The latter 3 have no impact on drivetrain wear. The only tool that measures chain stretch correctly is the Park Tool CC-4.
Nifty gadget as this gives you the exact elongation. But I think my $7.40 chain wear indicator from Amazon does the job. I just took this opportunity to check all my chains, and the one of my gravel bike has elongated over 1 mm. Not good.