I don't understand the benefit of a combat soldier carrying all that heavy gear on their backs. It seems that would interfere with agility and readiness. Please explain!
When you’re in a firefight, it’s too late to go back for anything you need, ammo, water, food, night vision gear, batteries, rain gear, cold weather gear, if you don’t have it with you when you need it you’re screwed. Watch “Blackhawk Down.”
You don’t know ahead of time how long you’ll be gone, how many enemy you’ll face, or how long support will be delayed. That’s why they carry so much stuff.
Different loads for different situations. Full “ruck out” of a mech infantry unit (assume vehicle down and you now have to move without it) can easily exceed 200 lbs.
Beans, bullets, water, weather gear, radios, batteries, body armor, helmet, weapon normally is in the range of 60 lbs. And you are right, it does interfere with combat, but that is not for the individual infantry man to decide.
Foot soldiers cross all sorts of terrain much of which may not have roads and thus need to carry their supplies with them. Weapons, ammo, food and water are heavy! (Not to mention first aide and med supplies, night vision gear, and mundane stuff like rain gear and rope and such.)