Programming the MCU is normally done via the debug interface (SW), but the USB/UART bootloader allows programming via USB or UART.
The EFM32 with built-in USB is shipped with a USB/UART bootloader programmed at the top of the flash memory. This USB/UART bootloader is designed to write data received via USB or UART to flash memory. This means that programming can be done from a PC via USB or UART.

Normally, user code is placed at the beginning of flash memory (address 0x0000), but since the bootloader is located here, you must specify an offset for the location of user code in the linker option when building. See AN0042 (USB/UART Bootloader) for detailed instructions.
Since the bootloader is located in flash memory, the size that can be allocated for user code is reduced accordingly. Overwrite or erase the USB/UART bootloader if you don't need it.