Final answer:
The default feature that becomes available when adding a custom object tab to an application is the 'Create New' functionality, enabling users to create new records for that object.
Step-by-step explanation:
When a developer adds a custom object tab to an application in a CRM platform like Salesforce, the additional feature that becomes available by default for the object in the application is the 'Create New' functionality. This allows users to quickly and effectively create new records for that custom object. It does not enable 'Quick create', which is separately enabled, nor does it add a 'Component'. While the tab itself does not create custom reporting, it does allow users to access data which could be used in reports. Lastly, 'Sidebar Search' is a separate functionality that may or may not be included depending on the design of the platform.