The core mobile structure, and the “tank” of the faction. One role is to serve as a Turret / Mortar target for a mobile structure. Because of it’s high life you can use the turret 2 or 3 turns usually at least.
Generally the main role is to serve as a “tank / blocker” to cover flanks and block enemy gates. Because of the high mobility they can reach those spots and then hold the position.
They are a permanent threat as they can do decent damage if the ability is used along with some upgrade(s). A good tactic is when you see a wounded Rust Rider, to pack them with upgrades and rigged explosives, send them to do damage, and then Detonate them.
Generally it is a bad idea to use their ability for just the +1 attack and this should be avoided.
Remember the Rust Rider as a structure can be built on the build phase on any place a gate could be built, which has a lot of uses, especially on defense. Or build it adjacent to Krusk so it can be moved for free next turn with Krusk’s ability.