The best way to understand the purpose of a cartridge is to use a real cartridge in a real system. A cartridge contained a game. Each system had one slot for a cartridge. You would insert the cartidge into the slot, then you would power on the system to play the game. The cartridge became part of the system; the system booted from the cartridge. To change games, you would power off the system, then you would remove the cartridge and insert the other cartridge, then you would again power on the system.
Each system had a unique type of slot, to prevent the insertion of a cartridge into the wrong system. Newer systems tend to use smaller cartridges. The Nintendo DS uses a small card instead of a cartridge.
With an emulator, a ROM image often takes the place of an entire cartridge; but a ROM image is less than an entire cartridge. The emulator must somehow decide the ROM layout and the amount of save-RAM. The emulators for SNES use the SNES header for decision.