One thing I looked at, was using them to mark Upgrades. In tournaments, you are usually required to have every piece of wargear represented on the model, but not all of use have the money to field 40 of every single loadout, and not everyone plays Tournaments.
Say you take a 10 man Imperial Guard squad as normal infantry one game, but try and use them as Vets the next. Obviously some wargear would not be on them (Carapase, Demo charge, and so on) so you could make markers reprisenting them to keep next to the squad so you do not spend a god awful amount of money on casual games. Also, even with normal Infantry squads, you may take Krak grenades, or maybe a melta bomb, so this also would work.