What to do with soul orbs?

I'm curious, what exactly are we supposed to do with these so-called "soul orbs"? Are they some sort of in-game currency or collectibles? Should we be saving them up for a specific purpose, or can we spend them freely on whatever we want? I'm just trying to get a better understanding of their value and what role they play in the overall game or system. Any insights you can provide would be greatly appreciated.