Half of 12 is 6, half of 1/4 is 1/8, I drill the hole at 6 and 1/8".
It's easy to get a fractional measurement of whatever is in front of you and immediately work with that measurement in your head then find the corresponding mark on your measuring tool. There is no reason the same system would not work with metric, it just seems to be more common with imperial units.
E.g. IKEA cabinets are 60 cm wide (90, or 120 for larger cabinets), common countertop depth is 60 cm, and so on.
That way we get to use sane and consistent units and we can easily see/measure the middle of a 60cm piece of wood or whatever.
It also tickles me how eyeballing these things is apparently something desirable. Whenever I work around my house I measure everything twice to be sure...
Not everything in the world is (or should be) provided by Ikea :)