If you are using the Fuel Rules in your Campaign the Fuel Depot provides a greater range to your vehicles allowing them to stay on the road longer or attack more distant Territories. Even in Games where the Fuel Rules are not being used a Force can benefit from extra Fuel in their Nitrous and Rocket Fuel Supplies.
Vehicles burn fuel to get them on the road and on Engagements, a Fuel Depot provides the Force with a resource of Fuel to keep them running, allowing they to make follow up attacks and reach more distant targets.
A Force with a Fuel Depot may add its Range to the maximum distance they can declare Engagements, and they may add the Range to their Fuel Tests during an Engagement. If you are not using the Fuel Rules then a Vehicle with Rocket Boosters or Nox has the Fuel Depot to supply them with more adding its Range to the number of Shots these systems have.