Because gaps like this multiply out at the end of a beam. If for example the abutting structural member relies on that joint for support and is 12 feet long (1

Because gaps like this multiply out at the end of a beam. If for example the abutting structural member relies on that joint for support and is 12 feet long (144 inches) and lets say the flange is 6" across, .005/6 x 144 = .12" which is about 1/8 of an inch of wiggle at the end. If your gap were, say .010" instead, there is suddenly 1/4" of wiggle and when things can wiggle like that vibration gets much worse.