I make extensive use of both Via and Shaping points in MyRoute.
Anywhere that I definitely want to go to, I switch to a Via Point (using the little hand icon in MyRoute). Anything else is automatically a Shaping point. I also make sure to name the Via points as clearly and simply as I can - i.e. "Lunch", "Morning Fuel", etc...
The XT then will definitely take you to each Via point, and try to route the rest of it using the Shaping points to the best of its ability. If traffic conditions change, it will then change the route between the shaping points according to your preferences - i.e. "Avoid Highways" etc..
Then when you import the route into the XT, you can choose your next destination when you tell it to GO - so you could start halfway along the route for example. You can also skip Via points while riding using the on-screen button. You can't start at or skip shaping points.
Via points are visible on the route as you ride, but shaping points are not. You can also select to display time remaining/distance/arrival time to your next Via point, which is pretty useful.
So the more Via points you use, the more hardwired the route will be to what you planned on the PC/Tablet/Phone
Tutorial here:
https://www.youtube.com/watch?v=hbRh2mHdmeM