Jesus, referring to The Holy Spirit, told the disciples just before ascending to heaven, the following "And, being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me." (Acts 1:8 KJV) "But ye shall receive power, after that The Holy Spirit is come upon you: and ye shall be witnesses unto Me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth” (Acts 1:8). The Promise of The father is The Holy Spirit.
Why we need The Holy Spirit presense in our lives:
1. He will lead us to all the Truth.
He will reveal the Truth, Christ Jesus, to us. "Howbeit when He, the Spirit of truth, is come, He will guide you into all truth: for He shall not speak of Himself; but whatsoever He shall hear, that shall He speak: and He will shew you things to come." (John 16:13)
2. He will help us please God.
The Holy Spirit in 2 Corinthians 4:13 is refered to as "The Spirit of Faith". Remember, without faith, its impossible to please God. He will help us to please God. He will show you and guide you into paths of holiness, paths of righteousness, all that are pleasing to God.
3. He will reveal to us things to come.
The Bible is 2 Peter 1:21, speaking of the Holy Spirit says "For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost" When He comes in our lives, He will give us gifts like prophesy. Paul in Corinthians admonishes to desire this gift. He says "Follow after charity, and desire spiritual gifts, but rather that ye may prophesy" (I Corinthians 14:1)
4. He will bring true fulfillment in Life.
When He comes into our lives, we experience true fulfillment. This is because His fruit becomes manifest. What is the fruit of The Holy Spirit "But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, Meekness, temperance: against such there is no law".(Galatians 5:22-23) Recall also that, The Joy of The Lord is our strength. We are not only fulfilled but also strengthened.