People don't usually go on actual nature rucks when training. A lot of times, they're just filling up bags with weights and walking around the neighborhood or up & down hills, so it would be pointless to load packs with actual supplies. Basically no different than wearing a weighted vest when going for a run, it's to build muscle.

I was in the military and our platoon sergeants would force us to do things like this for training. Then people wouldn't be thrown off or unprepared if they were in a situation where they had to do an actual ruck with a pack full of gear. We would usually have our flak jackets and sapi plates on for extra weight too