This has nothing to do with eyeballing, I hope I did not give that impression. In the imperial measurement world there are also standard sizes of things but that's not the point. Using a fractional scale (not necessarily imperial!) does have benefits in some situations, such as measuring and working with some arbitrary object placed in front of you. Measuring twice is not unique to the metric system and is very common in the imperial unit world as well.

Not everything in the world is (or should be) provided by Ikea :)