It works for solo games because of the order system where units on the map are given orders basically consisting of "move from A to B", which can be through a third waypoint, if you want a particular route taken, rather than just the line of least resistance. Berthier then works out the most efficient route for that unit to take, based on the unit's movement rates across the various different types of terrain it can pass though (or not, as the case may be).
This creates a certain 'fog of war', because the units will keep going for each campaign move until they reach their destination (which can take quite a few moves if the map is big and/or the movement rates are slow), and so enemy units may not be exactly where you think they are, as their actual position at any one time is worked out by Berthier, not by you.
|WW2 Operation Crusader|
|WW1 South China Sea|
There is one thing though, and I've not been cheeky enough to suggest this to Tony De Lyall, and that is it would be very useful to be able to set more than one waypoint for units. I was thinking this particularly for something like a commerce raiding campaign (or submarine hunting, etc.) where merchant ships or whatever it is you are looking for (or trying to avoid) effectively travel between two or more points (e.g. trading routes) that may take in a number of waypoints so that you really won't know where they are likely to be at any time.
A very good example of a waypoint setting system can be found in the (infinitely more complicated) Operation Flashpoint and ARMA series of computer games which come with an extremely powerful mission editor.