Yaakov Aveinu wished to ensure that even after his death his children would follow in the ways of the Torah. He gathered them together and suddenly lost his Ruach HaKodesh and the ability to tell them prophetically about the End of Days. He thought that perhaps one of them was incomplete with Hashem just as Avraham and Yitzchok had each bore one unworthy child (Yishmael and Esav) but they reassured him that “Shema Yisrael, listen our dear father Yisrael, Hashem is one.” At that time Yaakov responded with the phrase “Baruch Shem K’Vod Malchuso L’Olam V’Ed, blessed is Hashem’s kingdom forever” (see Pesachim 56a). This Gemara explains the connection between the first verse of Shema and the second phrase of Baruch Shem which we always say together in prayer.
The Gemara (ibid.) continues to ask why we only whisper the phrase of Baruch Shem when we say Shema in prayer. When Moshe wrote down the verses of Shema he omitted Baruch Shem, thus really we too should not say it. However, since Yaakov himself said it, we whisper it quietly. What in the world does this mean? To compound this issue, the Gemara gives a parable that is meant to clarify the entire matter which seems to only add to the enigma: There was once a princess who smelled a delicious aroma emanating from the kitchen. She asked her butler to bring her some of that dish. When he got to the kitchen he saw that it was not a royal meal being prepared for the king and his family but rather it was a private dish that the king’s servants has made for themselves. The butler reasoned that it would be disrespectful to present it to the princess for it was food of the simpletons, but on the other hand she asked for it and so he had to sneak her a portion without any fanfare. What is being said here? How and why does this relate to the phrase of Baruch Shem?!
Rabbi Chaim Volozhin (1749-1821, Nefesh HaChaim III:11) explains the depth here. The first verse of Shema establishes Hashem’s Unity and Perfection, there is nothing else to add to this. It states that Hashem is Intrinsically Perfect. Any further words would simply subtract. Hashem is the ultimately Perfect and Unified Being. Baruch Shem talks about Hashem’s rulership of the entire universe including earth and us His subjects. Imagine if a servant stood before the king and wished to make a request. He would begin by saying praises and recounting the King’s greatness. He would mention the King’s power, wealth, greatness and perfection. How would it sound if he would say, “You are the king of one million ant hills, one hundred thousand termite mounds and five million bird nests.” This may be true and that may be a part of his kingdom, but it is terribly disrespectful. It is the same thing with our mentioning of the praise of Hashem. When we say Shema Yisrael we are telling over the Perfection and Unity of God, this is the greatest praise. But when we say Baruch Shem we are referring to His kingdom of planets and earth, things that pale in comparison to His Intrinsic Greatness. We are talking about earth which compared to Hashem is a huge anthill!
So why do we mention it? Yaakov taught his children that Hashem in His Greatness and Infinite Wisdom created us to be His subjects and therefore He loves and cares for us. Although we are lowly in comparison to His Greatness, He put us here in order to serve Him, and praise Him and grow close to Him. That is the greatest pleasure to Him. This is truth was part of the revelation of Yaakov regarding the end of days. Ramchal (Taas Tevunos) explains that the more lowly and dark the world gets, the greater the revelation of Hashem’s rulership is manifest. Yaakov taught us that our job is to elevate our lowly state and to bring holiness and Hashem’s presence down on earth.