The only purpose of speech is to communicate. Herein lies the difference between thought and speech: thought is for oneself, while speech is for another. [Since kingship involves ruling over another], Malchus is termed Supernal Speech, for [like speech in comparison to thought] it is but an illumination. However, the illumination of Malchus emanates from a truly lofty source.
The matter is as follows: The intent of any and all G-dly comprehension is the benefit it brings to one's Divine service. This is also true regarding the general concept that the creation and vivification of the world and its creatures involves only a glimmer of G-dliness. The multitude of worlds and creatures were created by G-d with one word, as it is written: "Blessed is He who spoke [but one word] and the world came into being."
This thought is also expressed in the Mishnah that says:1 "The world was created with ten utterances." The Mishnah then asks why ten utterances were necessary, when the first sufficed to bring about creation. The answer given is that the last nine are merely an amplification of the first utterance of Bereishis.
One should also ponder how utterly trivial and insignificant the individual is in relation to all the worlds, from the highest spiritual world to the lowly physical one. Nonetheless, man is the ultimate reason for the creation of all the worlds, and so the heavenly angels and Supernal Sefiros wait impatiently for man's study of Torah. When one reflects on the fact that he is the focal point of all creation, he will apply himself more diligently to the study of Torah and the performance of mitzvos.
The Mishnah states it thus:2 "Each and every individual is obligated to say: 'For me was the world [olam] created.' " The word olam implies concealment, for the bodies of created beings conceal the souls within them. Accordingly, each person is obliged to say that G-d created the concealment in order for him to remove it and reveal the light of G-dliness through the study of Torah and the performance of mitzvos.
In summary: The Sefirah of Malchus is Supernal Speech, whose purpose is [to communicate with] another. The world was created through ten utterances, though one alone could have sufficed. Man is the reason and mainstay of creation, and the vitality of all worlds depends on his performance of Torah and mitzvos.