What is the benefit of purchasing a Mac formatted External Hard Drive?
It's probably no different hardware-wise, just already formatted for the Mac. There's a whole pile of people who still think Macs need special everything, so the companies take advantage of that to fleece them out of an extra £20. 

In general, you don't need special Mac-only hardware stuff anymore. Hard drives, RAM, peripherals, etc. are all fairly generally universally compatible; even the stuff that's generally seen as "Mac" (i.e., FireWire and now Thunderbolt) is Windows- and Linux-compatible and will generally work fine.

EDIT: I guess I can think of one good reason to buy a pre-formatted "Mac" drive: if the user is not someone who is comfortable formatting disks. On the other hand, Disk Utility is sufficiently straightforward I've walked my parents through formatting drives a couple of times over the phone, so I wouldn't waste the money.