Everyone needs a lightweight day pack and here is why:
Traveling: When flying, I hate checking bags so I need to fit everything into two carryons. It is easier said than done when traveling with fishing and camera gear. One bag has my clothes and camera gear and the other has fishing rods, reels, and flies. In order to adhere to the two bag policy, I compress my Flash 22 to the size of a grapefruit and stuff it in my carryon. When I arrive to my destination (or sometimes after boarding the plane), I pull it out, load it up with water, food, gear, and have a perfect day pack!
Hiking: Standard day packs are way too big. They are great overnight packs or specialty packs (if you’re hiking in winter or need more gear for some reason), but for most day hikes, a small pack is more than sufficient while minimizing weight. I do many fly fishing day trips and my Flash 22 holds water/water filter, food, an extra layer, sunscreen, and my fly boxes.
Commuting: I use my pack almost daily for bike commuting. It is light and comfortable and more breathable than my other packs. It easily holds water, bike patch kit and spare tube, a change of clothes, and the removeable back base prevents my 5lb Kryptonite U-lock from digging into my back.
* The REI Flash 22 model I like is discontinued. I chose this model almost solely for the deep mesh side pockets for quick access to water, food, rain gear.