some reason, so I can see how they might want to encourage someone to get to 10 then add the rest.
If I am adding 8 and 5, I might say "I can take the two from the five and now I'm at ten and I've got three leftover from the original five so I will add ten and three to get thirteen." It makes sense with helping students who can't memorize 8+5=13. It's a decent cognitive exercise.