If you enable "Calculate route between items", OsmAnd calculates to route to your destination using the intermediate waypoints you've specified. And as OsmAnd has calculated the route, the underlying map is used. From my experience and reports here on the board, the process isn't always perfect, but I'm glad it seems to be working for you. And it certainly helps that you've used OpenStreetMap to do the calculation.
If you don't enable the option, OsmAnd just follows the GPX without taking the underlying map into account. That's when you get a notification of every turn--a turn being simply a deviation of some minimum angle from a straight line. As not all GPXs contain intermediate waypoints, this may be the only choice.
At least that's my understanding. Corrections welcome.