Warhammer 40,000 – Daemon Engines are one of the unique things available to a Chaos Marine force. Most of the other vehicles they have are just the spikey equivalent to the loyalist counterparts. Daemon Engines are a unique blend of monster and machine.
The Decimator is a bigger version of a Helbrute, smaller than a Knight yet larger than a Helbrute it fits in a special place in Chaos lists. For my Night Lords I've opted to go with the dual butcher cannon load out. Eight strength 8 shots that reduce leadership by two can really put a hurt on some units. If I'm able to engage them with raptors or terminators after causing the wounds that penalty only gets larger and more effective at making the enemy flee in terror.