The system defaults to choosing the fastest route based on the selected mode of transport. But this doesn't mean that you can't change the route path shown.
Your scenario could be that you want to show
- The actual route you took
- A different route to go through specific towns
- A path that goes past a point of interest
- The scenic route, rather than the fastest route
Take a look at the article 'How to customise the route line path' for instructions.