They're kind-of lifestyle, in that they do strongly convey a certain "woo special forces oo-rah!" vibe (that I don't actually like -- though you can occasionally buy versions without all the MOLLE), but they do originate from being an accessory for a specific sport/exercise routine: rucking. I.e. long hikes in mud with heavy metal weights in your backpack. You don't need one of their bags to do it, but they're well-optimized for the task. Specifically: they're famously incredibly durable and last forever, and have good load-bearing traits so you can stuff 50lbs into them and expect to carry it about as well as possible. You can imagine how this translates well to less-demanding activities like walking through airports with all your clothes. :D
The main thing that might make you not want them is that they're very unstructured. They're a big empty box on your back with only minimal pockets, and if you want to impose any separation on gear you put in there you're going to have to do it yourself via other bags (e.g. packing cubes). This is in contrast to brands like Tom Bihn who have a bunch of specialized compartments to help you split stuff up. This is, of course, very much a matter of taste.
The main thing that might make you not want them is that they're very unstructured. They're a big empty box on your back with only minimal pockets, and if you want to impose any separation on gear you put in there you're going to have to do it yourself via other bags (e.g. packing cubes). This is in contrast to brands like Tom Bihn who have a bunch of specialized compartments to help you split stuff up. This is, of course, very much a matter of taste.