Shimano service instructions for hubs show that out of the four sides, three should be "forward-facing spokes are heads-in", and one, the drive side on the rear wheel, the other way around:

[![Shimano instructions for spoke direction](https://i.stack.imgur.com/fv4f8.png)](https://i.stack.imgur.com/fv4f8.png)

What is the reason to specify one way out of the two?