When you play on 2b you might need to extrapolate lines if you want to follow a perfectly strait line.

Usage could be something like this:

1. Set on point on your map
2. Set another point on your map
3. The map automatically extrapolates the line out and places a new waypoint in line with the two set waypoints. The distance might be configurable or the extrapolated line might display on the map.

I don't really get the utility of this, can you give an example of when this would be useful?