When a developer creates a new cartridge to implement site custom functionality in Salesforce B2C Commerce, it is crucial to ensure that this cartridge is included in the cartridge path of the relevant Storefront site. The cartridge path determines the order in which the system loads the cartridges, which in turn impacts how the site's functionality is executed. Adding the new cartridge to the cartridge path ensures that the custom functionality is accessible and executed as intended during the site's runtime. This step is essential for the new customizations to take effect and be reflected on the Storefront.