USB Interfaces

Phantom Power allows your condenser microphone to operate. Some other devices such as active DI boxes will use Phantom Power. If you're not using equipment which requires Phantom Power then it's not a feature you will find necessary, however virtually all USB audio interfaces come with Phantom Power functionality as a feature.

No. While there are some small audio interfaces which will run via bus power, interfaces with more inputs or other specific features will require a power supply to function.