Two leads are better than one

I bought a little 250GB USB hard disk. It comes with a strange thing I've never seen before: a split USB lead which goes into 2 USB sockets at once. Just as well I haven't got a MacBook Air with it's single USB socket!

Presumably this allows it to read from both or write to both or read from one and write to the other, or power through one and data through the other - who knows, Seagate certainly don't say.

Edit: Canadian Rob tells Robin it's for power, but you don't have to use both.