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Conventional drives have to be pressurized, so that the heads float above the platter. Traditionally, that's meant sucking air into the unit through a fine filter. Said filters can get clogged, reducing air pressure and causing the head to crash into the platter. They also allow some particles through, theoretically nothing large enough to do damage, but over time you can get junk floating around in there. A sealed drive filled with an inert gas eliminates that problem, at least until the gas leaks out. Then it crashes. 
 
Back in the day, cleaning and replacing hard drive air filters was part of my job. But with the advent of cheap drives the presumption was that the drives will become obsolete in a couple years' time anyway, and they stopped making the filters replaceable. And in fact modern drives are far more dependable than the $80,000 units I used to work on. Still, if you have a critical application that can't tolerate drive failure, you've got to figure out how to keep them clean and low-humidity for long-term reliability. And who worries about such things nowadays? Nobody.
 
Because there's no dust to worry about, platters in helium-filled drives need less platter-to-head clearance and can therefore be placed closer together, allowing for greater capacity. Does that make it worth spending 5x as much for a disk drive? In a laptop, yes, I think it might. In a desktop in a clean studio, nah. 
 
Back to topic...when they say "NAS-ready" what they're really saying is they're high-availability. It means they've got shock/vibration absorption. It means they're intended to be operated 24x7. Some are hot-swappable, which is essential for always-up server RAID arrays. These considerations are a big deal for, say, an online retailer or a 911 dispatch center, but not to a DAW user.