The Decimator is capable of being armed with either a singe Siege Claw or it can use two of them in tandem, turning the Decimator into an extremely effective close combatant. Decimator Siege Claws are equally effective against both "soft" targets such as enemy infantry and against hardened targets such as buildings and armoured vehicles.
The reason that these weapons are considered as "siege" armaments is that they feature built-in Heavy Flamers. The Daemon Engine can use these to engulf enemy infantry in flames at close quarters or to rip open buildings and armoured vehicles and instantly reduce their occupants to ash. This is a highly useful ability when assaulting positions defended by enemy infantry sheltering behind hard cover.
If a Decimator armed with Siege Claws is dedicated to the Chaos God Tzeentch, its built-in Heavy Flamers gain the ability known as "Soul Blaze." The Heavy Flamers are then capable of burning the soul inside of its target's physical body, consuming the unfortunate victim in clouds of ethereal fire.