This question has come up a few times over the past few weeks (both in person and through email messages) so I decided to sum up my reasons here to share.
I used to not wear a pack on day hikes or walks with the reason being that there was no need to do so--a water bottle could fit in a cargo pocket or be carried on the belt. Just go out and enjoy nature. There's some up-front advantages to that rational such as less time on preparation to go (although, let's face it--preparation is extremely important to safe hiking), less weight to carry, more trail time faster. All good things if that's what you're looking for. But that's not my focus right now.
When we did the Striper Cove modified loop last weekend, I got some looks and questions when we stopped to make hot tea before heading back to the car:
Are you rough camping on the trails? No, just hiking with it. The state park only allows camping in designated areas and while I can camp very low-impact (all the way to no-impact), I follow the rules and don't camp where it's not allowed. Unfortunately, far too many irresponsible and disrespectful people have made it worse for everyone else--thus all the rules.
Are you carrying a tent? No. I carry a hammock system. Do I have a tent? Yes. Do I pack it around? Not since deciding to give hammock camping a try--much lower impact and lighter to carry. Tree straps, a quality hammock, under quilt, top quilt, and a tarp weighs far less than a tent and sleeping bag.
What do you have in that thing? Not as much as I would on a backpacking trip but more than would fit in a knapsack. First aid kit. Radio (ham radio handy talkie). Sunscreen. Bug repellant. Once I decided to start carrying a full (or almost full) pack on my hikes, I have enjoyed being able to have hot tea (or coffee or hot chocolate, if you prefer) or make a meal on a break. Come up to a nice rest spot with a view? Break out the stove and pot and make a warm drink to enjoy it. Maybe a warm snack--enjoying a sunrise with your breakfast of scrambled eggs and sausage is pretty awesome.
Is that as heavy as it looks? I don't know--how heavy does it look? To be honest, I've never weighed my pack with a typical loadout. That being said, it is extremely comfortable to carry. The weight of a larger pack is carried on the hips via the waist belt. Every frameless pack or knapsack I've carried before has all the weight on the shoulders, which gets old VERY fast. An external frame pack like mine transfers that weight to the hips and uses the shoulder strap system just to keep the pack upright with an air gap between the pack and your back.
Wow! I haven't seen a pack like that in years! Where did you get that? I currently use an Alps Outdoorz Commander + Pack Bag purchased from Amazon. This is a 5250 cubic inch (~86L) external frame pack that I purchased a couple years ago for a trip that never happened. When Bear (my Lovely Wife) and I decided to get into backpacking, it made more sense to use the pack I already had instead of buying a new one. Don't get me wrong, I have plenty of knapsacks ranging in size from 20L up to 52L, but for carrying gear nothing compares to a suitable pack with a waist belt. I can't give advice regarding internal vs external frame packs, but I am a big fan of hiking comfortably...and a proper pack allows that to happen.
But WHY am I carrying it? To be honest, I need to get into shape and be healthier. My work is at a desk and outdoor exercise is preferable to indoors for me. I'd also like to get to a condition where I can start taking some backpacking trips along sections of the AT, which isn't far away. Getting used to carrying a pack and the added weight enhancing the exercise aspect are two big parts of it.
So there you have it. That's why I carry a pack on the trails while I hike, even for day hikes. What pack to do you carry on your treks? What works for you? Leave a comment and let me know.