why is normal force in the direction of vector normal?

This might be a bit philosophical, but is this just from experiment? that for a really really smooth surface, people found that the net force on an incline surface is found to be approximately in the direction normal to the incline plane? (approximately because there's no perfectly smooth surface, unless you use a magnetic plate i guess).


But are there any other reason, maybe just convention? that normal force is in the direction of normal to an incline plane?