It testifies of Christ. "ye may know that he is, by the power of the Holy Ghost" (Moroni 10:7)
It carries the gospel message into the hearts of those we teach (2 Nephi 33:11).
It tells us where to find investigators and how to do missionary work. Moroni taught, "the Holy Ghost led them whether to preach, or to exhort, or to pray or to supplicate, or to sing, even so it was done." (Moroni 6:9). It will tell us what to do as it did Nephi, " I was led by the Spirit, not knowing beforehand the things which I should do. Nevertheless I went forth." (1 Nephi 4:6-7). In 1832 the Lord speaking of Jared Carter, a missionary, said "the Comforter...shall teach him the truth and the way whither he shal go." (D&C 79:2). The spirit leads missionaries to investigators and helps teach them.
THrough it we remember what we have learned so we can teach it. Christ said, "the Holy Ghost ... shall ... bring all things to your remembrance" (John 14:26).
It builds relationships of trust with investigators. Peter said of the Gentile, "And God...(gave)...them the Holy Ghost, even as he did unto us; and put no differences between us and them" (Acts 15:8-9). They Holy Ghost prepares others to be taught.
It enables missionaries to teach with power. The authority in the Church is shown, Christ said, "ye shall receive power, after the Holy Ghost is come unto you" (Acts 1:8).
It makes missionaries and investigators want to do good. Those who heard King Benjamin said, "because of the Spirit...which has wrought a mighty change in us, or in our hearts, that we have no more disposition to do evil, but to do good continually" (Mosiah 5:2)
It enables investigators to feel they are being taught the truth. Investigators may forget the discussions or the scriptures but they will never forget what they felt when the missionaries taught them. Having once felt the spirit it enables missionaries to make return appointments to teach and bear testimony.
It guards missionaries from temptation. Individual worthiness leads to spiritual power and protection. The companionship of the Holy Ghost provides protection against the adversary.
The Doctrine and Covenants teaches, "And the Spirit shall be given you by the prayer of faith; and if ye receive not the spirit ye shall not teach" (D&C 42:14). A prayer of faith is different from ordinary prayer. Faith as Alma taught is not a perfect knowledge but lets a person act as if they know something is true. Thus faith is the power to act without perfect knowledge. We tell the Lord in prayer what we are going to do and make a commitment to Him - then we GO AND DO what we told him. When we ACT LIKE A MISSIONARY and GO AND DO what we have promised the Lord, we are blessed with the spirit. In the DOING the spirit comes. This is why faith is often referred to as a PRINCIPLE OF POWER.