Haven't been around long, have you?
Ignore the DC input. If you know anything about PCs, USB, and HDDs, you'd know exactly why it's there on a USB-powered HDD.
USB-powered hard drives operate outside of powered USB specifications. Even within spec, not all USB ports are powered, and I don't just mean those from an unpowered hub. Apple notebooks have long shipped with unpowered USB ports to sell their iSight firewire web cams (though they claim that it is to save batteries of something). Users of these notebooks will either need a direct power plug or a medusa cable that draws power from either A: additional ports or B: a power plug (often a USB-charger wall plug).
Just like Western Digital USB-powered 2.5" drives, the auxiliary power is optional.