This not for more realism but for more enjoyable/interesting game mechanics to play with: When reloading, you can either drop the magazine to save some time in the reloading process, but you'll loose the remaining bullets, or you grab the magazine and put it back in your mag pouch, which will make the total reload time a bit longer, but you'll get to keep the remaining bullets from the magazine. This way depending on the situation you can either do the fast tactical reload when survival is at risk or the responsible reloading to make sure you have enough bullets left for the next fights. It will feel more rewarding and immersive (small things that make you choose and think, require your attention for a moment makes you feel involved and present in that world)