To make a sketch of a frame so that the tube ends actually converge in the right places, length accuracy down to 1 mm is helpful. For drop, rake, trail, radial wheel spacing and lateral spacing between chain stays (tire clearance), millimeter accuracy is essential since these are such small numbers.

If you measure with an inch ruler, saying "23 in" for example suggests exactly 23." However the rules of rounding numbers say that it could really be between 22.5 and 23.5, for an uncertainty of 25.4 mm! Saying "22 1/4" suggests exactly 22.25" where the right way to signify that is to say 22.250."

"Right way" implying common engineering practice - engineers need to care about what the precision is and have developed ways to convey that information. If you want to use numbers to calibrate a sketch to reality, you need to be cognizant of accuracy.