"Not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching." (Hebrews 10: 25)
This verse could not be clearer.
We get together as believers in the Body of Christ for one reason: to exhort, to encourage one another.
The word "exhort" is παρακαλέω, parakaleo, the same word which references the Holy Spirit.
"But the Comforter [Parakletes], which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you." (John 14: 26)
The Holy Spirit witnesses of one person: Jesus!:
"But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me:" (John 15: 26)
How then do we find encouragement? In Jesus, and when we come together, we encourage one another in our growing knowledge of Jesus. Church was all about celebrating what Jesus did:
23For I have received of the Lord that which also I delivered unto you, That the Lord Jesus the same night in which he was betrayed took bread: 24And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me. 25After the same manner also he took the cup, when he had supped, saying, This cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me. 26For as often as ye eat this bread, and drink this cup, ye do shew the Lord's death till he come." (1 Corinthians 11: 23-26)
We also come together to comfort one another with the truth:
"Finally, brethren, farewell. Be perfect, be of good comfort, be of one mind, live in peace; and the God of love and peace shall be with you." (2 Corinthians 13: 11)
When false teachers were claiming that the resurrection already passed, Paul spoke the truth:
"16For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: 17Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord. 18Wherefore comfort one another with these words." (1 Thessalonians 4: 16-18)
Peter exhorted his fellow believes to grow in grace and knowledge of the Lord, too (2 Peter 3: 18)
We come together to encourage and be comforted by our advocate Christ Jesus in our lives, for He is our comfort, the Word made flesh, the way, the truth, and the life:
"I, even I, am he that comforteth you: who art thou, that thou shouldest be afraid of a man that shall die, and of the son of man which shall be made as grass;" (Isaiah 51: 12)