A familiar feature that most Pokemon Fans will also quickly take note of is their bag. The Bag allows a trainer to keep a number of extra items on hand as they journey through their adventure in Pokemon GO. These items include pokeballs to catch new Pokemon, Potions to heal your hurt Pokemon after a gym Battle, Revives to bring back any of your Pokemon who may have fainted in Battle, Incenses to attract Wild Pokemon, Berries to aid in catching Pokemon during encounters, and Egg Incubators to help hatch the Pokemon Eggs you find during your journey.
Like many of the newer Pokemon Games, you can actually hold a lot of items in your bag in Pokemon GO. You can actually hold a grand total of 350 Items when beginning the game. You can even go into the Shop and buy 50 Extra Items Spaces in your Bag if you need it, having up to a 1500 Max Capacity. No matter how much of an excess this might seem to be, it does come in handy the longer your journey becomes.
The bag might not seem as essential when you're off playing and catching Pokemon, but if you didn't have it, there would be no game at all. And remember to keep track of your items and the quantity you have of each. Nothing is worse than running out of items, especially pokeballs, during your journey. If that happens, you'll quickly find yourself having to trek out to numerous PokeStops and waste a lot of time to slowly, one by one, collect more items. That, or using your real world money to help restock your bag.