Answer: Because you can. :-) Happily and safely. It can also open up an entirely new world of possibilities for you. For those who have had to give up backpacking because of bad backs or knees that couldn't handle heavy loads anymore, using ultralight approaches gets you into the wilderness again. If your body is holding up fine, going ultralight also allows you to greatly expand your options on the trail. No longer limited to 10-12 miles a day (unless you choose to be), you can see more, walk more, explore more if you wish. Or just sit around a nice basecamp and not have sore shoulders from the hike in. Or carry more food for longer trips.
I used to carry 40+ lbs for weekend trips, 60+lbs on longer ones. With that load, I could comfortably travel 8-10 trail miles a day, in variable terrain. Since I typically get to backpack only 2-3 days at a time, I could only get so far away from the trailhead. The two or so longer trips I could squeeze in every year were great, but I felt there was far more "out there" that I just wasn't getting to see. And in popular national parks, it seemed impossible to get away from the crowds.
Of course having an ultralight load doesn't mean you HAVE to cover any more miles than you usually would with a heavier load. It simply makes it possible for you to do so, IF you choose to. If your goal is to simply hike 5 miles to the nearest campsite, go for it! With an ultralight load, you'll have lots of energy left over to explore a nearby peak while your heavy-load partners are recovering under a tree. :-)
I just happen to really enjoy walking, so the more time I can spend wandering through the wilderness, the better. When traveling with heavy-load friends (and hence spending less time on the trail and covering fewer miles), I have more energy to explore our surroundings, either during the hike or after we set up our campsite.
By traveling ultralight, I can now see and experience far more of the backcountry that previously I just couldn't get to during my weekend trips. Traveling ultralight, I can comfortably walk 15-20+ trail miles a day, and get away from the crowds faster while seeing more of the "good stuff".
It's not just that I can travel farther in a day, but I am also far more comfortable, alert and happy while I'm walking, so I tend to observe far more than I ever could before. Instead of sweating and grunting like a beast of burden, plodding along and staring at the trail, I'm looking and listening all around me. I see more. I hear more. I experience more. Trail time is now "quality time", not a period of pain and suffering to be endured between campsites.
Don't equate ultralight backpacking with fastpacking. This is an entirely separate issue from having an ultralight load, but I think the topic is worth discussing here since having an ultralight load makes it possible to happily cover the extra miles. As other long-distance trekkers will point out, it doesn't necessarily mean you travel FASTER to cover those miles, you simply travel LONGER at the SAME SPEED. And those hours spent walking are enjoyable ones, taking in the wilderness as you go, stopping to watch a marmot scurry around, or swallows darting around the cliffs.
On my long days (20+ miles), my average pace is just over 2 miles/hour, including rest breaks. Note that 2 MPH is the average speed for most every backpacker, REGARDLESS of load carried. The only difference is that I amble on the trail for 10 or more hours (including breaks). Longer, not faster. Of course there's nothing wrong with fastpacking if you enjoy doing so, it just doesn't happen to be my style. I'm slow, but I'm persistent.
Interestingly, I find that I often notice more of the wilderness than a lot of folks, even though (or perhaps because?) I spend so much time on the trail. Remember, trail time to an ultralight backpacker is a great chance to take in your surroundings, seeing stuff that many folks just grunt through on their way to the next campsite. On my "rest" breaks, I always have energy to explore off-trail, and have managed to find many a perfect spot that I would never have stumbled across otherwise.
One person's cult is another's salvation. *8-) Seriously, I have yet to meet a backpacker who started out ultralight. Well, except for Justin. :-) (Hi Justin!) We're all converts to one extent or another.
The reason so many of us are jumping up and down about it isn't due to a "look at me" mentality. Quite the contrary, many of us were simply stunned when we realized how comfortable and safe you can be while carrying a load that we'd been convinced was impossibly light! We're jumping and hollering to let everyone else know that it REALLY can be done, and you don't have to be a mad hermit to do it! Though being a *little* nuts couldn't hurt any.... *8-)
Fifteen years ago, if someone had told me their base pack weight (i.e. everything but food and water) was less than 10 lbs, I would've assumed they were either a dangerous nut, a terrible liar, or both. Yet here I am today traveling with a true 3-season 2-5 day base pack load (before food/water) of less than 10 lbs. Ray Jardine and his wife have thru-hiked the Pacific Crest Trail with base loads as light as 8.5 lbs!
Of course any rational person should realize that what really counts is individual happiness, and to that end I don't believe ANYONE has the right to "look down" on others simply because they prefer a different approach to backpacking (assuming we're all low-impact, right? ;-) ). Everyone has different styles. All I (and I presume other ultralight promoters) want to do is make people AWARE of the possibilities. Most folks won't ever even consider it as an option if they're convinced it's not possible. Well, it's possible! And it may work for you.