If you had to walk through terrain unsuitable for a bike, or if you wanted to carry it on a plane, train etc. It would be nicer to pack inside something rather than having all the protruding bits scraping and catching on things. I'm not suggesting anyone treks through the Amazon rainforest with it but foldies do need to be carried from time to time, and the most convenient way to carry something heavy is on your back... well, actually the most convenient way to carry something heavy is to get someone else to carry it, obviously.

It's also got mini wheels so it can be pulled behind like a wheely-suitcase, with or without the bag.