Typically, you can only use/save/edit/etc a single palette at once.
The ability to be able to load multiple palettes at once would be useful for situations where we want to keep certain palette files separate normally, but also use them together in select sprites. An alternative would be to merge the colors into a single palette, but that gets messy, and means that any changes to either the original palette file or the merged palette file need to be copied over to the other.
Also made a small mockup below - nothing fancy, but it gets the job done. Same as the current palette UI, but with a ‘-’ button to remove palettes and a ‘+’ on the bottommost palette to add a new one.