Real world outside the military? Not many?
Some civilian ideas(?...just making some ideas up here...?):
1. Used to show new shooters flight trajectories
2. Mythbusters-style testing, does XYZ catch fire from tracers
3. "Mark" a target to have others fire on
4. For long range shooting, seeing how they drift at extreme distances can help judge windage beyond the muzzle (if other indicators aren't present).
Note, I've done #4 once since my spotter couldn't see the bullet impacts for my first shots. Although the tracers aren't as accurate, it at least gives you a better visual of what is happening down range.
Others?
-Rhino12
^^This. For rifles with no last round hold open (like CETME/HK91/G3 and .308 AK variants) they can be loaded first into the mag to indicate time for a reload.Another idea I have heard is loading your last xx amount in a mag so you know when your low, good for tac-classes, your HD mags, not sure if comp-legal though.