There isn't really a compelling reason to run significant storage off 2.5" HDDs anymore -- before SSDs 10k or 15k 2.5" HDDs made sense (access time), but nowadays...
Here's a compelling reason: cheap, portable mass storage. Eg. USB3/Thunderbolt-powered multi-terabyte drives that fit in a backpack and don't require external power.
A 3.5" drive fits in most backpacks, and it can be powered of USB 3.
Not easily. Even an energy-saving drive will probably draw 15 watts of spinup current.
Though of course you can fit a 12v/2a adapter in the backpack next to the drive.