@poperamone: I think it's more that you don't necessarily need to use the inventory to complete the game. There's a few utility features, like you can go onto the move list to see what fighting moves you can do, there's a calendar to check date, you can look at photos you've picked up, you can use the save and resume feature, you can see all your capsule toys, you can use the cassette player to play tapes, learn moves you've picked/bought and so on.
As for items you buy, dried fish/milk etc can be fed to the cat. Capsule toys are just collectibles so you don't really need to use them. Chocolate/Caramel/Crisps are a bit pointless - but buying them gives you a lucky draw ticket.
zafir's comments