[There are about 6 classical books that come under this title Musar. It is not an open cannon but already fixed.] [חובות לבבות, שערי תשובה, מסילת ישרים, ספר היראה מיוחס לרבינו תם, אורחות צדיקים, מעלות המידות]/
One argument is given by one disciple of Israel Salanter, Isaac Blazzer. He brings down from the Rambam that Musar is a cure for all mental spiritual and physical illnesses.
Reb Nachman brings down that fear of God is beneficial for length of days. [That is--to have long days in which you do not have to waste your time doing meaningless stuff. After all, the best way to spend one's life is to find the objective meaning and purpose that is already inherent in it. Not to put meaning into it. And to find the purpose of your life and to do it, is usually so hard that it takes almost one's entire life to find out what that purpose is.
The point of learning Musar is not to find out what good character is. It is a tool to try to work on oneself in order to develop good character.
The whole idea of Reb Salanter was to use learning Musar as a kind of service in itself. Not as a way of acquiring information.
In terms of the importance of good character a being one of the most essential goals of the Torah, there is the Reshash (Shalom Sharabi from Yemen and then from Yerushalaim, the Rambam in the Guide concerning the reasons for the mitzvot, the Chafetz Chaim, and other sources. They are all unanimous in the ultimate importance of good "midot" good character. [This is one good reason to avoid the religious world-- as they parade good character, but in action do the opposite. ]