There's 2 types of items we use. Wieldable items, and Icon items.
#1. Wieldable items - These are items that you can actually hold to help your team with (Examples: Spyglass, Rangefinder, Spanner, Rubber Mallet, etc)
#2. Icon items - These are items that are never seen, and only you can use them while mounting a specific object like a gun or the helm. (Examples: Kerosene, Moonshine, Greased rounds, Burst rounds, etc)
The Pilot utility belt contains both iconic items for his specific job, and wieldable items that other classes can use.
The Gunner utility belt contains iconic items for his specific job, but no wieldable items that other classes can use.
The Engineer utility belt contains wieldable items that other classes can use, but no iconic items specifically for himself to use.
The gunner needs some wieldable items to make himself more useful, as he only really needs 2 ammo slots 90% of the time, and the engineer could possibly use some iconic items to boost engines possibly.