I think it is important to know how the values of flight parameters are calculated to give an understanding of what is going on. Hence exam questions like these.
Once that has all been understood and one is on the line in the cockpit though, rules of thumb are normally sufficient.
RoD for a 3° slope ~ 5 times the groundspeed, which as bafanguy mentioned, is much more easily worked out mentally as; Half the G/S, then add a zero.
It is important to have an idea of the approximate RoD expected; to guard against capturing false ILS glide-slopes and other gross errors.