Two problems here for you to figure out.

It's very useful to compare similar positions with different correct moves. In Position A we could play 24/15* 24/21 which gains in the race and keeps the back men connected. But the problem with that is we have no home board and black will have no difficulty entering and hitting us back. We end up in a fight where the luckiest player wins. It's better to make the two best available points on the board, the 21-point and the 5-point. Solid play.

In Position B, after making the 21-point there isn't really any other good move. 13/10(2) is just bad. As we already hold the 5-point we can hit the checker on the 15-point.