What kind of use cases are UEFI applications actually good for?

Groups of applications that make sense in UEFI:

• Configuration tools - Configuration interface for Option ROMs (ie. for storage controllers), out of band management (ie. AMT configuration tools), manufacturer performance tweaking tools
• Provisioning tools - used by administrators to preload specific BIOS setting, manually setting all options in BIOS setup would be inefficient
• Diagnostics tools - mostly for tests that cannot be performed in OS (DRAM tests, full storage scan, storage R/W tests, etc.). In some districts specific diagnostics tools are required in UEFI BIOS, so those can be sold to government.
• Security applications - HDD encryption/decryption, antivirus scanner and anti thief applications
• BIOS capability enhancement - Power Over Ethernet extensions, DRAM discovery, patching and modification of system tables (SMBIOS, ACPI)
• Display tools - for displaying complex animations while running, splash screen displaying
• Bootloaders - this is special type of application, which can call EFI_BOOT_SERVICES.ExitBootServices() causing termination of all memory management and passing control to Operating System.

Note that very important feature of UEFI application is that it can be added to boot order and be executed each boot time. Also UEFI application do not have to be delivered with BIOS image it can be stored in connected device memory, which is common for Option ROM configuration tools.