In practice it's difficult to make difference between:
- Forcing move vs. thank-you move.
- Abandoning useless stones vs. making move of equal size in other way.
- If playing light causes sacrifice, was the whole thing profitable or not?
One of core principles of the way stones work is that possibilities mean reduction. This is quite easy to demonstrate in endgame context. Once white descends with 1, black would have to play three stones at "a", "b" and "c" to stall the advance. Black is bound to lose a lot of points. I believe this had relation with defense and efficiency.
1 is a necessary move. How to tell? Is it just about liberties or number of empty intersections towards each direction?