This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
Why, yes, it does, as a matter of fact. It does nothing early in the
game, but has some nice gear-specific options later in the game.
For all gears (optional): Fix Frame HP - becomes active when the gear is
equipped with a Frame HPxx accessory. The xx determines how much HP the
frame fixer will restore as a percentage of your gear's total HP, but
the more powerful accessories also consume more fuel as well. Note that
using it outside of battle will always restore 10% of the gear's HP.
For Brigandier and Andvari: Ygg. D-charge - becomes active after Bart's
first underwater battle with Ramsus. Using it will call the Yggdrasil,
which will drop depth charges on all the enemies.
For Vierge: Aerod - becomes active after Dominia is defeated under the
sea. It does non-elemental ether damage to all enemies.
For Weltall-2: System Id - is active upon acquiring Weltall-2. It
immediately puts Weltall-2 into infinite attack level (see article
For Weltall-2: Thor Wave - is active upon acquiring Weltall-2. Despite
the name, this option does a non-elemental ether attack.
For Weltall-2: Hellfire - is active upon acquiring Weltall-2. It does
fire-based ether damage to all enemies.
For Renmazuo and El-Renmazuo: Jessie's Cannon - is active after
defeating Bishop Stone, after Fei comes to senses during disc 1. Jessie
fires a giant cannon which damages all enemies.
For El-Renmazuo: Jessie Blasta - becomes active after the omnigear
upgrade. It's a more powerful version of Jessie's Cannon, but it uses
more fuel as well.
Seibzehn uses a number of special options which can normally be used by
using Maria's "Controls" ether abilities. Specifically, Seibzehn begins
with a missile launcher, and unlocks a cannon after its first battle
Also, note that the Yggdrasil IV, used by Bart in the battle against Ft.
Avalanche during disc 2, has a special option called "Yggdrasil Cannon".
It does 9999 HP damage, but costs 9000 fuel units to use. Still, the
effect is worth watching.
Note that other than Fix Frame HP; Weltall, Xenogears, Heimdal, Fenrir,
Stier, El-Stier, and (of course) Chu-Chu do not acquire special options.