Let's be clear. Packs are for carrying stuff. The things in your pack should make you feel happy, safe, secure, and full. Each hiker must choose their comfort level. This is my take on when and why you should take a certain pack... and why I have so many packs!!
The Daypack
This pack type will consistently get more use than all other packs. You'll use it to go on the day hike, to go to the beach, to go traveling overseas or to festivals. Basically a man-purse (but for women too), but much more... manly (or womanly)!
If you pack correctly, the day pack can be used for a weekend overnighter too. Super UL and you'll have room to spare in your gunny.
The size comfort and cost of a day pack is really negligible. I wouldn't stress about this at all. I have one that was ~$90 with padded straps, bladder pouch, cinches, waste and chest snaps. And I have one that packs down to a small size with no frills at ~$25. Both work exactly the same - fits on the back to carry a snack.
Shoe pairing: Lightweight tennis shoes, flip flops, dress shoes, bare-feet!
The Weekender
The weekender might surprise you. I'd go for anywhere from 10 to 40 lbs. So a day pack could work if you're going for speed and distance, a 45-60 L pack to test out gear, or for winter camping, or bringing that 6 pack of beer or bladder of wine, and packing in your kids or girl-friends gear just to be chivalrous.
Miles are low, and sore shoulders expected, so I see no reason not to bring some trial gear, chairs, whatever. Just realize, it'll be painful!
Shoe pairing: Trail Runners for sub 35 lb loads, boots with ankle support for packs of greater weight.
The Thru-hiker
I would go with a ~20 lb pack or less if you can. If you are carrying gear day after day for weeks or months your body can't handle extra weight, you have a higher chance of injury for every pound extra, you have a higher probability of quitting. Besides rain gear and first-aid, if you're not using something everyday or every couple days, get rid of it or bounce box it ahead. A 24 - 30 L pack is all that should be needed. On the AT, and I would hazard any Long Distance Trail in the east, you can easily resupply every 5 days or less. If it's easy to re-supply in less than 5 days, carry less food. The store is a better place for food than on your back!
This is a comfort balance. You could send home the stove and go cold. You could send home the sleeping bag and just use a sheet. You could send home the tent and hope there is always shelter space. And if you do all those things you'll have a really light pack, and it might be enough for you. Or you could be prepared for many different situations, like the desire for a warm meal or coffee, or a desire to be warm on a cold night or have a roof over your head when miles away from a lean-to. You can still have a really light pack and have all those comforts.
Shoe pairing: Trail runners
The Expedition
Ya know those behemoth packs that every outfitter in the world has dozens more of than anything else? The 65-75+L packs? The ones that 97% of outdoor enthusiasts don't need. But they can charge the most for them, and people seem to love to buy them. Outdoor stores are doing a disservice to people by selling and promoting those packs for non-specific purposes. Take a person that never has backpacked. They walk into REI and the sales guy sells him a Baltoro and enough gear to fill it, so upwards of 50 lbs and $2000, this person puts this enormous load on, back creaking before ever stepping foot on the path. But, "hey, this is what backpacking is supposed to be, this is how it always looks on TV" and they groan and get blisters and are forced to wear huge heavy expensive ankle supporting boots. The get blisters, they get back pain, they put that gear in the attic and forget about it. That's not a good business model. You get someone a right sized pack, so they can enjoy those views they climbed so high to see, you get someone lightweight gear, and there's a much better chance they'll come back again and again for more stuff.
When will this pack come in handy? When you are going to Alaska or Canada and have no resupply points for 2 weeks. I see no other use for actual backpacking with a pack this large. Maybe to put rock climbing gear in, ropes and such the capacity would come in handy. Other than that, it's a scam.
Shoe pairing: Boots with ankle support
