Binyomin was the only tribe of the Jewish people that was born in Eretz Yisrael. He represented the completion of the Jewish people, as he was the twelfth of the twelve tribes. The number twelve always represent the completion, in the physical realm, as there are twelve edges to a three dimensional cube. As the Ramchal explains, Binyomin was the one who would serve to connect the aspect of Yosef and Yehudah, as we see in the story of the brothers, as well as in the persons of Mordechai and Esther.
It is interesting to note that while Binyomin was the only tribe to be born in Eretz Yisroel, Yosef’s birth was the impetus for Yacov to return to Eretz Yisrael, as there was no longer a need to fear Esav (as per Rashi on the verse of Yosef’s birth). The Ramchal says that Yosef includes Binyomin, as it says “יוסף לי בן אחר” – Rochel’s name for Yosef included a prayer for another son, which was Binyomin. Thus, we could say that an aspect of Yosef was included in Binyomin, and vice versa. This would explain Yosef’s special love for Eretz Yisroel that earned him the merit of being buried in Shechem, as well as the title עברי, Hebrew (as opposed to Moshe who was referred to as מצרי, Egyptian, and was not buried in Israel).
Yosef classically represents the aspect of Yesod, and it could be said that Binyomin, as the completion of the twelve tribes, also represents the aspect of malchus. These two aspects are always intertwined. This could also explain why the first king of the Jewish people was specifically chosen from the tribe of Binyomin. Binyomin is the aspect of malchus, and he is also the tribe born in Eretz Yisroel. The purpose of the king is to solidify the Jewish people’s claim to the land of Israel, and to rid the world of the forces of evil, embodied in the nation of Amalek.