I had the same question when I first got into road biking, and kept using my camelbak for the first year or so. It was highly convenient for storage and all the hydration I could ever want to carry, and didn't seem like that big a deal (do all the people talking about back sweat not come back from long summer rides completely covered in sweat, regardless of what they had on their back?)

Then one week I finally tried switching to bottles and a top-tube bag, and immediately saw a notable difference in my Strava hill climbing segment times.I think mostly the backpack was restricting my upper body positioning in ways I didn't realize until I ditched it. It could be bogus, but I think I use my upper body more effectively when I'm pack-free.