Without items, including the all important pokeballs, a player cannot succeed in Pokemon GO. Items aren't just a luxury, but a necessity in the success of a trainer within the game. Just about every item in the game can be obtained for free through PokeStops and Leveling Up, but a trainer can still buy a number of them from the Shop that aren't or when in need.
Some of these items - such as pokeballs, Berries and the Camera - are used when in an Encounter with a Wild Pokemon. While other items like Healing Items, eggs, egg Incubators, Lucky Eggs and Incenses are used during regular gameplay. Yet even a few items have special uses: Lure Modules are used by connecting to a PokeStop to increase Pokemon Spawns, Evolution Items help a select few Pokemon evolve, and the Bag and Pokemon Storage Upgrades happen automatically to their given areas after purchase. (Always be wise when choosing exactly when and where to use a given item.)
Some items, truthfully, aren't needed, but they do increase a trainer's chances at success in their own given ways. However, a number of items, such as PokeBalls and Healing Items, are absolutely needed if a player expects to win often in Pokemon GO.