Deathrain crisis suits are any suits that are armed with twin-liked missile pods and any combination of other systems. Some players choose to fill the 3rd slot with a target array, giving the suit a BS4 which all but guarantees that you'll hit with each shot. Others choose shield generators for a 4+ invulnerable save, but I choose the humble flamer. The flamer is the cheapest option and it gives the suits some bite as units get real close. Last Thursday I was able to flame some Orks with 23 flamer hits. You may decide other systems suit your style of play and I believe it makes little difference in the unit's role on the tabletop. It's the twin-linked missile pods that are important.
5th Edition is all about mech and Deathrains are very effective at destroying rhinos, Ork trukks, and others. Against AV12 they have a chance but don't count on them to wreck chimeras and wave serpents. Twin-linking the missile pods means that you don't have to rely on markerlights to hit. Leaving then markerlights for other units that need their guidance more.
Their range is also important as you can move away from units and still pump out the same amount of shots, as opposed to plasma rifles which tempt you to move closer to get in extra shots.
I generally keep them close to my vehicles. They jump out shoot and then jump back behind the vehicle to claim a 4+ cover save or even to block line of sight. You could also deep-strike them behind Imperial Guard Tanks and wreck those tanks from the rear. If you're doing this though I might suggest to equip with shield drones or shield generators, as they will most definitely take some return fire.
In my army, they have become a standard unit in any size games. Once you start fielding them, you'll know why.