Why are fragments used if Activity already exists?

Fragments allow creating modular and reusable user interface components within an Activity.

Main advantages:

• UI Flexibility: Easily create different UI configurations for various screen sizes and orientations (tablet vs phone).
• Reusability: The same fragment can be used in multiple Activities or multiple times within one Activity.
• Lifecycle management: Fragments have their own lifecycle, which is related to but managed separately from the Activity's lifecycle.
• Navigation: Convenient for implementing complex navigation flows within a single Activity (e.g., with Navigation Component).
• State management: Allow preserving part of the UI state during configuration changes or navigation within the Activity.

Example of usage for a tablet (master-detail):

Left panel (list of items)	Right panel (details of selected item)
ItemListFragment	ItemDetailFragment

Example code for adding a fragment to an Activity:

// In Activity
supportFragmentManager.beginTransaction()
    .replace(R.id.fragment_container, YourFragment.newInstance())
    .commit()