Rav Shmuel de Ozeida writes that, if Mordechai learned of this plot through prophecy, of course he had to do something with that knowledge. After all, one does not learn information through prophecy for naught.
Midrash Panim Acheirim posits that Mordechai reported this plot for three reasons:
By getting in the good graces of the king, Mordechai hoped to win permission to rebuild the Temple.
More generally, being liked by the king, Mordechai would be able to have influence for the sake of Jewish causes.
More practically, he had to do this in order to not be blamed for this plot. Although this would be neither the first nor last time a Jew is scapegoat in political intrigue, this is especially true according to the Ma’amar Mordechai’s opinion mentioned before that the plotters had originally attempted to sway Mordechai into joining their conspiracy.
The Ben Ish Chai brings from the Midrash (Bireishis Rabbah 39:12) that it is the duty of a Jew to save the world. Jewish advisers to foreign kings throughout our history have rescued them from impending doom whenever possible. Mordechai could not turn his back to this ancient tradition. Furthermore, writes the Ben Ish Chai, anybody would do similarly, at the least in order to avoid suspicion. The Maharal and the Me’am Loez both quote the Mishnah (Avos 3:2) that a Jew should pray for the peace in the government because anarchy and unrestrained progressive change can be dangerous.