“Hashem said to Moshe and Aaron in the land of Egypt. This month shall be for you the beginning of the months. It shall be the first month of the year.” 12:2-3
This [i.e. the obligation to proclaim the new month] is the very first Mitzvah that Hashem commanded the Jewish people through Moshe. Therefore it says, “in the land of Egypt” as opposed to the other commandments, which were commanded on Sinai. - Ramban
The mitzvah to proclaim the new month contains many deep symbolisms. On a simple level however, it points to the matchless ability of the Jewish people to rise up from oblivion and restore itself to past greatness. Just as the moon fades away at the end of each month only to restore itself to its former greatness, so too, the Jewish people will rise up again no matter how many times they fall. History is replete with stark examples of this phenomenon distinctive to the Jewish people.