In writing the Zero Armor's stats (under 000), I found that there were more purposes than (under Primary Purpose/Secondary Purpose) than typically described in Rifts books. Thus, here I relate more:

Quick Kill: This is a weapon used for a decisive kill; a singularity generator or anti-matter cannon would fall under this category. It's not necessarily the most efficent weaponry, but it sure gets the job done.

Environmental Damage: This is two things: the ultimate form of anti-personnel and just attempting to cause ultimate damage. Think about it: when you're attacking a base, don't you just want to rain shots and destroy it? Who gives about accuracy; make it a crater, right? Also, when you fire a machinegun, you may not necessarily be doing environmental damage (which can be argued), but you sure as hell are filling the air with bullets trying to make the environment damaging.

Trap: This is a sort of modifier; weapons like Hagan's fusion blocks and the NG Juicer ATV's mine system can fall under this category.

Intimidation: This one is typically a Secondary Purpose. So what if a weapon is not as effective as it could be? An intimidation factor can definitely be useful. Quick Kill weapons are usually Intimidating.

Timesuck/Buying Time: This is a less common one, but it is still applicable. Suppose you had a Kittani Plasma Net, and you had to fight two enemies. One enemy was merely a minor irritation, and you wanted to be able to stop worrying about him. Couldn't you use the Net to "get rid of him" and focus on the other enemy?

Auxillary Weaponry: Sometimes, you just want another gun. This is a purpose that is typically Secondary, and basically means that if it is not a main weapon, but an emergency weapon, is used. Eye lasers are much like that; ammo considerations can also be taken into account. A minigun would probably not be Auxillary, but a laser weapon might be Auxillary just because it has unlimited ammo and can be turned to when the minigun runs out.

Automated Weaponry: This is far less used in Rifts, but sometimes there will be independent combat computers with their own melee attacks. These automated weapons can be a life-saver for a pilot.

Total Destruction!: In the Invid Invasion book, the Synchro Cannon is entitled to be Assault, but I truly doubt that. A weapon that obliterates everything in a 50x10,000 (width by length) foot radius? That's not Assault anymore. Total Destruction is like a combo of Quick Kill and Environmental Damage.