One use case is to estimate the minimum latency for communications between two points on opposite sides of the Earth, which would be how long it takes light to travel halfway around the Earth:
= 20,000 km / (speed of light)
~= 66.7 milliseconds
= 20,000 km / (speed of light)
~= 66.7 milliseconds
Beams of neutrinos can be sent through the Earth and detected on the other side.
https://www.livescience.com/19075-neutrino-particle-communic...
(diameter of Earth) / (speed of light) ~= 42.5 milliseconds
Some high-frequency traders might actually be able to justify the investment needed to enable such low-latency communications!
Come to think of it, you could also use gravitational waves...