Watch the video tutorial on on-route.com and all will be revealed, but in summary:
This software is much better for finding scenic roads when planning a route, MapSource can avoid highways and favour small roads, but where a pass is small or especially twisty, MapSource will often decide that the road is too long and direct you another way. On-Route, has a take ''scenic roads'' option which is more than a simple avoid, this builds routes that are near to features of interest (forests, lakes, and roads with many turns), it is complimentary to MapSource, but it does much more of these type of things.
On-Route generates options for your routes that you can choose between and compare directly, it also has a very cool 'excursion wizard' which allows you to plan and preview day trips from and returning to the same location with various options.
In addition to that it does more with pois and has lots of pre-generated routes built in, so think of it as an easy way to plan better trips.
It's true that MapSource is already a useful product, but On-Route builds on the best features of MapSource for bikers particularly and so is a logical extension to it for those left wanting.
That's taking it from a purely Garmin point of view, but on-route is also very useful for TomTom (where it extends the possibilities of route planning enormously) and also allows non-gps users to benefit from many of the cool features of a GPS without having to spend the money on one.
I can't really do all of the features of the product justice here, so watch the tutorial if you want to learn more, you might even recognise the voice!
HTH,
David