Most .308 box magazines are relatively cheap, light, simple and hold 20 rounds. They work great for those who wear additional magazine pouches and have friends cover them during reloads. Drums are not cheap, weigh a little more and hold 50 rounds. 100rd double drums are much heavier and work best for fixed positions.
So what do you get for the extra money spent? 2.5x as much ammunition before the first reload is necessary. Machined feed lips that are resistant to damage. Thick convex mag body, also resistant to damage. For a single fighter who has no one to cover his reloads and who might not have a load-bearing vest on, it makes sense. You pay more for the magazines but win time in combat.
Unlike extended box magazines, properly designed drums do not put every round under constant spring pressure, only those inside the feed tower. That improves feeding reliability. A drum is also shorter than a 30rd .308 magazine, so firing prone is easier.
This particular design does not require tools for loading.