Well I think that's the idea. It lets you choose related movies based on specific aspects of the seed, so that what it returns is as well-tailored to your appetite as possible.

Just giving a seed is much more difficult (see: Netflix Prize).

The UI doesn't make it clear, but if you don't want to use the Nanogenre thing, use the 'most like' bit on the left. I think the UI would be improved by making the Nanogenres and 'most like' more equal, as they are both useful but in different ways.