BAIS YOSEPH
The Tur in Oirach Chaim Siman 188 writes that one should mention “Malchus Bais Dovid” in the 3rd Brocho of benching (Boneh Yerushalayim).
The Bais Yosef comments on this (188:4)that if somone does not mention “Malchus Bais Dovid”he has not fulfilled his obligation.
He goes on to discuss the details of the Brocho and then concludes with an explanation of why we need to mention this and shows how we also mention it 3 times a day during the Amidah prayer. He says the following: Chazal say that on the day that the house of Dovid split (during the times of Rechavam) the Jewish people rejected 3 things: Hakodosh Baruch Hu, The Bais Hamikdash and Malchus Bais Dovid. They will not be redeemed until they demand all 3, Thus it was enacted to say the 3 Brochos in the amidah
1) Mishon Umivtach Latzadikim, Malchus Shomaim-Hakodosh Boruch Hu)
2) Boneh Yerushalayim (Bais Hamikdash)
3) Matzmiach Keren Yeshua (Malchus Bais Dovid)
We see from here how important it is to demand the Geulah!