Example 3. (Zu Chongzhi Cup Mathematics Competition) The sum of all digits of the natural numbers $1, 2, 3, \ldots, 9999$ is $\qquad$
Solve $0+9999, 1+9998, \cdots, 4999+5000$
all sum to 9999 without any carry, so the sum of digits is more:
$$
4 \times 9 \times 5000 \div 2-5=89995 \text {. }
$$

This is called the method of integer value combination.