In Singleplayer, this serves as a "favorites" and is a safeguard against carelessness. Getting rid of ammo, meds, or w/e you may want to keep because you were spam clicking or otherwise... human.
In Multiplayer is where this shines, as you currently cannot put anything in a car trunk without it becoming public property. Once back at the base, the host can simply go to the trunk and warp all your hard earned loot right into the base without your permission. And with no way for you to get it out. Well, mark it as yours, and no one will be able to pull it out of the trunk but you. If the host deposits it, it will be sent to your Storage Locker, not theirs. In this way, you, the owner, keep your things, but the host doesn't have to worry about trolls or pranksters filling up their car space with items they can't move.
Optionally, the host should also be able to drop any loot on the ground. As it is their game at the end of the day, and they need to have the final say about what comes in the car and what doesn't. Plus, it serves as another method of preventing trolls from filling up trunk space with marked items.