Because all the digits are the same, each digit is worth 10x the one to its right and 1/10 of the one to its left.
This is because we count in base 10. This is also why we can expand this number to become
9*10^6+ 9*10^5+ 9*10^4+ 9*10^3+ 9*10^2+ 9*10^1 +9*10^0.
Hope this helps!