Changing an auto-increment value is useful when you don't want a gap in an AUTO_INCREMENT column after a massive deletion.
For example, you got a lot of unwanted (advertisement) rows posted in your table, you deleted them, and you want to fix the gap in auto-increment values. Assume the MAX value of AUTO_INCREMENT column is 100 now. You can use the following to fix the auto-increment value.
ALTER TABLE your_table_name AUTO_INCREMENT = 101;