What good does registering an item do on Pokemon sapphire?

You're likely thinking about the PokéNav, which allows you to register Pokémon you've encountered in the wild. This doesn't have any direct effect on the Pokémon itself, but it's useful for a few reasons:

* Tracking: You can see where you last encountered that Pokémon, making it easier to find them again.

* Information: You get basic information about the Pokémon like its type, height, and weight.

* Dex Completion: Some players like to use the PokéNav to keep track of which Pokémon they've caught and which they still need to find.

Here's how the PokéNav helps with your Pokédex completion:

* Seeing which Pokémon you haven't caught: You can see a list of registered Pokémon, and those marked with a question mark haven't been caught yet.

* Remembering which Pokémon you've already caught: If you're trying to complete your Pokédex, the PokéNav helps you avoid wasting time catching Pokémon you've already registered.

Overall, registering Pokémon in the PokéNav is more about convenience and organization than affecting the Pokémon themselves. It's a helpful tool for anyone wanting to complete the Pokédex or find specific Pokémon.