In some ways katate dori is the source of all attacks. Predominantly we practice it because it is easier to feel the contact an blend.
However, it is easy to cut someone down to the floor (if they are of similar size) by grabbing their forearm and cutting it down as if with a sword (one or two hands, as long as you direct the elbow).
Also, forcing someone's arm down can keep the near leg from kicking (since the uke's weight is forced on to it).
Probably most importantly, we all assume that techniques are done when someone attacks, but I'm certain that originally it tended to be the other way around i.e. you start by grabbing someone, then putting a technique (particularly pins) on them (ikkyo, sankyo, nikkyo, shiho-nage).
Other than blending exercises we always use katatedori as if it is a real attack i.e. the opponent is forcing them to the ground or the other hand is coming in for a strike (pulling you on to a punch).
I've also been grabbed by agressors more times than I have been directly punched in real life.