Saint Thomas Aquinas, in his sermon on Pentecost, explains why the Holy Spirit was sent to us:
I say, first, the Holy Spirit is sent without His needing to be sent. When someone is sent to a place so that an event may happen which could not happen unless he were sent, this would be a sending out of necessity. But this has no place in the sending of the Holy Spirit, whom the Book of Wisdom describes as “having every power, beholding all things” (Wis. 7:23). What, then, is the reason for the sending of the Holy Spirit? Our neediness; and the necessity of this neediness of ours comes partly from human nature’s dignity, and partly from its deficiency. For the rational creature excels other creatures because it can actually reach the enjoyment of God, which no other earthly creature can do. “The Lord is my portion, said my soul” (Lam. 3:24). Some seek their portion in this world, such as those who seek worldly honor or dignity. But the Psalmist says: “It is good for me to cling to God” (Ps. 72:28). You should consider that all things that are moved to some end must have something moving them toward that end. Those that are moved to a natural end have a mover in nature; but those that are moved to a supernatural end, namely to the enjoyment of God, must have a supernatural mover. Now, nothing can lead us to our end unless two things are presupposed, for someone is led to an end by two things—knowledge and love. The kind of knowledge in question is supernatural: “No eye hath seen, nor ear heard, nor hath it arisen in the heart of man, what God hath prepared for those who love Him” (1 Cor. 2:9). “Never have they heard, nor perceived with ears, nor has eye seen, O God, without Thee, what Thou hast prepared for those who await Thee” (Is. 64:4). Now, whatever a man knows, he knows either by discovering it himself or by learning from another. Vision serves discovery and hearing serves learning, and for this reason it is said that “eye hath not seen, nor ear heard,” showing that it [the final end] altogether transcends human knowledge. It exceeds human desire, too, and that is why Scripture says: “nor hath it arisen in the heart of man.” How, then, is man led to know it? It was necessary for heavenly secrets to be made known to men; it was necessary for the Holy Spirit to be invisibly sent, in order to move man’s affections so that he may tend toward that end. And thus it says: “Eye hath not seen.” How, then, do we know? “God hath revealed it to us through His Spirit. For the Spirit examineth all things, even the deep things of God” (1 Cor. 2:10). “Who would be able to know Thy thought [sensum], unless Thou gavest wisdom and sent the Holy Spirit from the Most High?” (Wis. 9:17). Therefore the Holy Spirit is sent not owing to any need of His, but for the sake of our benefit.