Suppressive fire is mostly a niche action. There's a few reasons to use it.
1) Miniguns get x1.5 DV on Suppressive Fire. You can get some serious mileage out of the Vindicator LMG with SnS or APF ammo. The vehicle mounted miniguns can annihilate people.
2) If you only have 1 IP and little or no RC because you are a mook, Suppressive Fire is way, way better - it "lasts" a full combat turn, but your enemies can trigger it on each action, and you don't take recoil for using it.
3) Capsule rounds are very effective with Suppressive Fire, because they don't get DV from net hits anyways.