It’s called additive manufacturing whereas you only use the material you need.
With filament, it looks like weed eater line, it is extruded into much smaller melted plastic that hardens in place, building a part.
With resin, you start with a liquid that is hardened with light layer by layer.
You don’t need near as much material as you would with starting off with a block of aluminum and removing what you don’t want. And you don’t need all the tooling and machines required for manufacturing.
It’s best used for one-off applications and very low volume production, currently. The cost isn’t competitive with mass production.
That’s fine for parts that don’t require any significant strength. You won’t find 3D printed mechanical parts on airplanes.