There are only really three things that can utilize that much bandwidth as found in thunderbolt 3:
1) Graphics - Like low latency high resolution display esp multi-monitor or external graphics card (note you can get USB 3 multi-monitor external graphics cards and docks, they just don't do 3d so much)
2) Hard drive, esp external SSD
3) Networking esp wired high end fibre connections
I think of those two, the most likely use cases people might want are eGPU, and fast external harddrive. So basically turning your laptop into a desktop by docking it, and playing with mixed reality or games, or storing masses and masses of media and connecting to a big screen for personal media hording enjoyment.
Neither of those are typical use cases, but they are also things I'd probably want to do. There again usb 3 of any kind is plenty fast enough for external media hard drives etc anyway, and most wired connections will also do fine with normal usb 3.
Really the best use of thunderbolt 3, would be all three - external harddrive, external GPU running to high res screen and high speed wired network connection.
Note that, usb c is not always thunderbolt 3. If you get USB c on an ARM chip, or last gen intel, its not as fast as it is on the latest Intel.