Explanation:
The phrase is put before the last element of a list indicate that although the thing (or person) is mentioned in the last placed it does not mean that the person is the least important.
Example:
"I would like to thank: Paula, Ann, Marie and last but not the least Christie."
The phrase 'last but not the least' is used to tell that Christie although mentioned in last place is not less important than all the other people mentioned before her.