"“Vertical or horizontal?” — when setting a piece of text in Japanese, this is a question that Japanese designers constantly need to ask themselves. Being able to use both vertical and horizontal writing orientations is something so normal for us native Japanese speakers that most of us won’t even stop to wonder why this is possible, or even when and how it was first introduced."Essentially they're able to read both and they're used interchangeably or more often to emphasise context. This aids in the production of such things as tube maps because allowance don't need to be made for orientation of labels.
A Japanese poster for PiXAR's new film Brave also shows the versatility of having vertical and horizontal text.