Meaning of heading parameter for via routing

The heading in 1st point is more important for navigation purposes to start driving in a specific direction.

For the rest points according to documentation:

“It is important to note that if you force the heading at via or end points the outgoing heading needs to be specified. I.e. if you want to force “coming from south” to a destination you need to specify the resulting “heading towards north” instead, which is 0.”