You need access to a wide variety of different weapons and items during your ascension through Exit the Gungeon. The more diverse choices you have at your disposal, the more creative gameplay you can have and the more damage you can inflict. It takes a bit of time to unlock additional choices, though, and they all come from one location, which you need to unlock.
Before you can unlock the characters behind the shop, you need to obtain the Rat key, which you can grab after defeating your first boss. It’s the floating gold key in the corner, underneath Ocelot’s shop who provides you with gear and helpful items in-between your Exit the Gungeon fights. With the key, you need to continue through the run until you reach a brief cutscene with Cadence and Ox. After you save them from their entrapment, you can now find them on the left side of the Breach.
Inside their shop on the left side is where you can purchase new items. On the right side is where you can purchase new weapons. You do not gain immediate access to these toys. Instead, they enter the random drops and potential options you could acquire during your game. You need to play Exit the Gungeon more to have a chance of encountering them and seeing if they work for you, or not.
The more items and weapons you have the potential to roll during Exit the Gungeon, the longer you can potentially last the more damage you can do. You want to visit these two often in their shop to see if they have anything new available for sale. They only accept hegemony credits, which you receive after taking down a boss.