One thinks about design at the oddest times and places. When on a hiking trip in Iceland a few weeks ago, I had planned that we eat snacks constantly throughout the day, but when stuff was in our packs we tended wait until we could find a sheltered spot where we could take the packs off and get at the insides. (Sitting on this icefield was actually one of the most protected spots that I could find). A really good designer will look at how things are actually used and adapt to those circumstances.
The Camp compartment (1) is for all the gear you won't need until you roll into camp, such as your tent, sleeping bag, cooking system, and food stash. By keeping all the heavier and bulky items close to your center of gravity, you will have better balance over uneven terrain and a more comfortable ride. As water is also heavy, an external hydration sleeve is positioned at the back of the pack but close to the top for easy access.
The Trail compartment (2) is for all those extra layers you don't immediately require but might need at some point along the trail, such as your rain jacket or warmer layers, headlamp, maps, and snack food. The large slot pocket on the front is great for stowing your water shoes for those inevitable creek crossings. The On-the-Go section (3), with its two roomy side pockets and hip pockets, is for all those items you want while hiking, such as water bottles, camera, lipbalm, GPS, and snacks.
It seems so logical to do it this way, to divide the pack up according to functions; I couldn't even reach my water bottle in my Deuter pack. Another demonstration of the benefits of good design. More at GearCaster.