In Ocarina of Time, there are various places (as an adult) that Poes appear. You can fight and kill them, like any other enemy, but when they die, they leave behind a glowing "soul" type thing that you can put into a jar. Why would I bother? They don't appear to be really good for anything. Is there something I am missing here? What do I do with the souls?
From the Zelda wikia:
This is slightly incorrect — I spent about an hour killing/drinking Poes and only observed 3 effects (in order of frequency): +1 heart, -1 heart, and full health restoration. On the whole, soul-sucking is usually beneficial.
As well, there are 10 Big Poes throughout the game:
Here's a guide to their location, should you need it.