so you can monitor the descent profile.
Or indeed, as we do, use the ALT/DIST scale to place yourself on, and stay on, the 3° slope to the runway. As it is a straight-in, you need to be AT the MDA right on slope, otherwise you'll be high on the VASI and) depending on your aircraft type and manoeuvring ability) you probably won't get in.
We actually use the 300ft per nm system as it's simpler ot anticipate on the altimeter, but the principle is the same. With a distance-to-run interrupt at Foxtrot, life becomes much more difficult.