To allow your mobile device to be used as a wireless remote camera.
To pull in other people's video and audio for podcasting/broadcast (guest appearances)
For sharing high-quality and low-latency audio and video across the Internet and within LANs.
Bring a friend's remote game stream into your OBS and do side-by-side gaming together.
To allow you to publish to YouTube with your smartphone even though you don't yet have enough followers to broadcast to Youtube with the YT mobile yet.
To watch movies with friends, via screen sharing, privately, and with low-enough latency to talk on the phone together while watching it.
Use as a remote low-latency teleprompter feed.
Recording remote or local video at high quality without needing any downloads.
Remotely streaming MIDI device output, such as MIDI keyboards or production control boards.
For whatever other reason you might come up with.