It's intended to teach how the final digit of a sum of one-digit numbers that adds up to more than 10 arises. You're supposed to split up one of the numbers to make the other one add up to ten, so 8 + 2 + 7 (which is what the picture suggests) and 9 + 1 + 7 would be valid solutions, and both show that the overall sum is 17.
These funny ways of teaching mathematics keep failing, however. People need to understand that there's never going to be an "easy" way that lets all the children just whizz through with no effort to get to understanding what numbers are about.