Here is my real-world example of an auto-routing vs manual creation:
Let's build a route from Solomons, MD to Salisbury, MD. The Wicomico river is a very winding river. This route is approximately 50nm vs 38nm in a straight line. I have built this from scratch as well as with the autorouting on the Navionics app.
I use the Navionics app just to create routes(which generally takes about 30 seconds to create), but that then requires an export to a .gpx file, that in turn needs to be imported on my TZ iBoat app on the tablet. From there, I fly over the route to make sure that the exported route is actually in navigatable waters, save the route with a proper name and wait for it to sync to my displays before departing the dock. In this example, there are 283 waypoints created to make this a smooth route going up the Wicomico river. I would say there are probably 100 waypoints that are not really needed on this auto-generated route. This entire process depending on how long it takes for the cloud to do its sync averages 7 mins. Manually creating this route took me almost 25 minutes.
If I was just navigating out of a harbor and just going straight to the fishing grounds out front and it only involves 3-10 turns, yes I agree that manual creation would be faster. At the end of the day, it is still up to the captain of the vessel to ensure that the course is in safe navigable waters.