But I(Jesus) tell you the truth, it is profitable for you that I should go away; for unless I go away the Helper will not come to you; but if I go, I will send Him to you.
John 16:7
Holy spirit was already with the disciples
Eg if I say I will come to your house and stay forever, next time I cannot say I will come because I'm already there !!
The Comforter is the Holy Spirit of God, the Spirit of Truth.
John 14: 16 And I will ask the Father, and He will give you another Counselor to be with you forever. 17 He is the Spirit of truth. The world is unable to receive Him because it doesn’t see Him or know Him.
But you do know Him, because He remains with you
and will be in you. 18 I will not leave you as orphans; I am coming to you.
Keep in mind that Jesus is speaking to His disciples and that His disciples know the Holy Spirit, but this time the Holy Spirit will be in them -- Greek "en" -- indwelling Jesus' discples.
As to why Jesus had to go away in order for the Comforter to come, I think commentator John Gill gives you the answer:
"The Spirit of God in some sense had come, before the death of Christ; he had appeared in the creation of all things out of nothing, as a joint Creator with the Father and Son; he was come as a spirit of prophecy upon the inspired writers, and others; the Old Testament saints had received him as a spirit of faith; he had been given to Christ as man, without measure, and the disciples had been partakers of his gifts and graces; but he was not come in so peculiar a manner as he afterwards did; as the promise of the Father, the glorifier of Christ, the comforter of his people, the spirit of truth, and the reprover of the world: there are reasons to be given, why the Spirit of God should not come in such a manner before, as after the death of Christ. The order of the three divine persons in the Trinity, and in the economy of man's salvation, required such a method to be observed; that the Father should first, and for a while, be more especially manifested; next the Son, and then the Spirit: besides, our Lord has given a reason himself, why the Spirit "was not yet given, because Jesus was not yet glorified",
Joh 7:39; And the coming of the Spirit as a comforter, and the spirit of truth, was to be through the intercession, and by the mission of Christ; and therefore it was proper he should go away first, in order to send him; add to all this, that if Christ had not gone away or died, there would have been nothing for the Spirit to have done; no blood to sprinkle; no righteousness to reveal and bring near; no salvation to apply; or any of the things of Christ, and blessings of grace, to have taken and shown; all which are owing to the death of Christ, and which show the expediency of it: the expediency of Christ's death for the mission of the Spirit to his disciples, is very conspicuous; for hereby they were comforted and supported under a variety of troubles; were led into all truth, and so furnished for their ministerial work; and were made abundantly successful in it, that being attended with the demonstration of the Spirit and of power.