It's rarely necessary to hollow an FFF print, especially if you're going to perform casting as part of the project. You use the infill to provide for appropriate support of the overall model as well as to provide for a smooth upper layer surface.

If you're going to use a model as a coin bank, you'd want a hollow inside, but you'd also have to ensure the design provided support for the skin while it was printing.

One can save on filament by using lower infill percentages. Resin is many times more expensive than filament, hence the desire to have hollow models in that process. Another practical reason is if one prints a non-hollow model on a resin printer and the inside is uncured resin, later mechanical failures with the model will result in leaking resin. It's considered a hazardous substance, and you don't want the risk. Filament is pretty low risk in comparison.

The cura wall thickness value is valid but provides for skin strength and the system designs the infill from that reference point.