The issue is that sometimes a cable will appear to stretch as the housings seat properly into the cable stops, so being able to adjust the cable later on, can be handy as it's only a millimetre or 2 shorter that it needs to be. The other issue is that bicycle rim brakes are not self adjusting, so as the pads wear down, the brakes move further from the rim. Again twisting the adjuster to adjust the cable shorter moves the brake closer to the rim, keeping the same braking function as the pads wear. I don't know of anyone who cuts a cable off right at the brake, you always leave a centimetre or so to add the cable end.