Very simply a scant 1/4" is necessary for rotary cut projects so all the pieces fit together as they should. That means no fudging, easing or lost points. A scant 1/4" is merely 1 or 2 THREADS narrower than an accurate 1/4". Believe it or not there ARE times when something like 2 11/16" is an accurate measure; and you are VERY likely to come up with that same measurement if you used Mary Ellen Hopkins Personal Piecing Measurement Method.
The trick is to learn when you need to use it and where you don't have to.