It’s meant for measuring objects and that’s exactly what it does. When you need to measure something ‘in a pinch’, Ruler App is there. The makers of Ruler 2 are also having a contest - “In celebration ...

iOS: Sometimes you need to get a quick measurement of something and you probably don’t carry a ruler around, but of course, there’s an app for that.

Microsoft is working on a Screen Ruler tool for PowerToys. The utility has various modes and can measure the distance between objects.