I saw a dude riding his road bike with a large backpack on. He was riding on one of the local bike trails so I know he was training and not doing some cross country thing. I thought maybe he was just strange but later I saw several more people doing it…….including a guy on a TT bike. What is the point in doing this? I mean these were not small packs but the large camping/hiking packs.

because it makes you work harder and also feel stronger without it on. like swinging two bats when warming up.

Baseball players swing two bats to loosen up not to make one bat feel lighter..... And putting weight on your back will not make you work harder.

probably a commuter. i've been doing it for years.

I think I mentioned this on the roadbike forum.
A very good Cat 2 racer (he can sprint with the best) would ride all Winter with a water bottle loaded with pennies. (he had it tie-wrapped to his frame and cage so it wouldn't rattle)

He claimed it made him feel sooo much stronger and faster come race time. (he removed the bottle just a couple weeks ago)

Anyway I am not sure of the actual merits of added weight, but I wouldn't want to be adding strain to my BACK by carrying it there. If I was going to "add" extra weight, it would be as this guy - Bruce does in a water bottle and/or with heavier wheels/gear.