There are a few in-game ways of using Supplies pretty fast though:
Producing items using your Assembly Plant(s)
Trading with other players
Use the Expert Settler Perk, this can cost up to 5% of your maximum storage capacity of Supplies if you use stacked captures
Constructing and upgrading Concrete Walls, Assembly Plants and Warehouses at your Bases and Outposts also requires many Supplies
Each Soldier costs you one Personnel and one Supplies so go out and fight the AI