What a great question!
The bible presents the role of the Holy Spirit as a continuation of the work of God the Father and Jesus, the Son in the lives of the believer. As such He is an essential companion of every Christian and is the guarantee and means by which we persevere to the point of reaching heaven. Let me explain.
When God rescued Israel from Egypt he did so so they would become his people and he would become their God (Exodus 6:6-8). This was fulfilled at Mt Sinai when God met with his people and gave them the law. They camped around Mt Sinai (Exodus 19ff) and God dwelt with them, teaching them about how they were to live as his people. This concept of God dwelling with his people is a crucial aspect and in many ways this meeting can be seen as the first “church” of the bible. God continued to dwell with his people first in the tabernacle (read the rest of Exodus especially Ex 40:34-38) and then in the temple (1 Kings 8:10-11). God dwelling with his people was always a part of God’s plan. Indeed God’s greatest act of judgment was to withdraw from his people when their sin became so great (Ezekiel 10) and he sent them away into Babylon.
When we come to the New Testament, John describes Jesus in a very evocative way. In John 1 Jesus is described as God’s Word who is God and who became man and dwelt with us. Here John sees God coming to dwell with his people again. Jesus did all that God had done at Mt Sinai - telling people about God and about how they were to live as his people. He gathered around him his disciples who became the first “christians”. On the eve of his death, he gathered with them and spoke to them about his departure (John 13-16). As you can imagine they were grief stricken. Jesus promised them another companion (or counselor) to take his place and who would be with them forever - the Holy Spirit (John 14:15-31). The Spirit will dwell with (in) them (vs 17-18), and teach them and remind them of Jesus teaching (vs 26).
This is, of course, what happened at Pentecost (Acts 2) with astonishing results. The disciples were turned from scared people cowering in a room for fear of the Jews into the people who courageously proclaim the news of Jesus despite all that happens to them. The Spirit in them enabled them. It is by the Holy Spirit that God (the Father and the Son) dwell with us, his people, and continue to teach us and help us to stay true to him. That is why we need to Holy Spirit.
As such the role of the Spirit is multi-faceted.
Using the bible, the Spirit teaches and convicts us of the truth and assures us that they are saved (1Jn 3:24, 2Cor 1:22, Eph 1:13; 4:30)
It is the Spirit in our lives that unites us to Christ in his death and resurrection (Rom 6:1-11, Gal 2:20). Being ‘in Christ’ by the Spirit secures our completeness (Col 2:9-10). Being ‘in Christ’ by the Spirit unites us with others (1 Cor 12:12-13, Eph 4:4-6)
He empowers us to do what is right, live in faith and do good works (Ro 7:18, Eph 2:4-10). It is always God’s work in us that means that we are saved in Christ and living new lives for him. It is the Spirit that makes Christians aware of their sin and empowers them to turn away in repentance.
It is the Spirit who distinguishes us from unbelievers (Eph 2:12, 1 Cor 15:21ff) and empowers us for mission in the world (Acts 2:1).
The Spirit is an essential part of the Christian life, the Spirit is how God works in his people until the second coming of Christ. The Spirit helps Christians to understand the word of God and live it out in their lives today.
There is much more that can be said but I hope this paints an amazing picture of what we have been given in the Holy Spirit and encourages you to read and find out more.