It's for when you're laying out stud locations on top and bottom plates. You're using the speed square to square the lines for the studs, and you have a stud that's doubled, like a king stud and a jack stud together, You draw the line for one stud, then you slide the square forward 'till that line is at the 1-1/2" mark on that scale, then you draw another line along the marking edge, which gives you another squared line 1-1/2" from the first, without having to lift the square from the plate, and use scale along the marking edge to measure 1-1/2", (or pull out your measuring tape, or grab a piece of 2x scrap to use as a spacer) then lay the square back down to draw the line. Works for making pairs of squared lines from 1" to 3-1/4" apart, which not coincidentally, is the width of a 2X4. There are probably other layout tasks where this feature is handy.
