As the population starts seeing the goodness of having on-demand access to *many* TV episodes and movies (by which I mean their entire DVD collection plus video bought online - in our case, 300 movies & 1000 TV episodes), they'll start filling up their existing drives.
Once you've migrated your 500+GB iTunes media library from one drive to another more than once, you start not wanting to do that again. At that point, an infinitely expandable option starts to look very, very attractive.
Combine that with the gibbering night sweats that come from realising how much effort it'll take to re-rip all your media, or pay to replace from online stores, and drive redundancy becomes a must have.
Device full, but you have spare bays? Slam in another drive. All done.
Device full, but no spare bays? Pull the smallest capacity drive & replace with a larger one. All done.
Drive died? Pull it & replace with working one. All done.
No downtime required, no complex rebuilding of arrays, any mix and match of SATA drives will do the trick.
It's about as close as you can get to a consumer suitable appliance for high volume, safe, data storage.