In a game like Pokémon, having access to items is kind of mandatory. You’ll need Poké Balls in order to capture Pokémon, potions to heal them, and revives to wake them if they fall during battle at a Gym. There is a store to handle some of your item needs. For now, you’ll receive a daily allowance from Professor Oak, but that may change when the game leaves Beta. Currently, the store does not carry a handful of items only found at Pokestops such as potions, revives and razz berries.
You never know what you’re going to come across at a Pokéstop but it’s always helpful. Each item has a specific use in the game. For the most part items fall under recovery, and capture. That is, items either help your Pokémon to recover from wounds, or are one of a variety of Poké balls used to capture Pokémon. There are also incense to lure Pokémon to you, Lucky Eggs which grant you double experience for 30 minutes, Lure Patches which will draw Pokémon to a Pokéstop, Razz Berries which make it easier to catch more of the same species of Pokémon and eggs which can hatch into new Pokémon.