I thought about getting one, but more of the "bucket in front" Netherland style. I'm too lazy to go shopping once a week, when I'm going to fill that thing up to the top.
The ones that ride on the front wheel mean you have to shift the load for all steering, for even the slightest correction. Inconvenient, and dangerous even.
It appears it's just an additional gear to keep the chain from hanging as low and loose as traditional chains to help prevent things, like pant legs, from getting caught in it.