The book of Acts tells us that on the day of Pentecost, following the death, resurrection and ascension of Jesus into heaven, the Holy Spirit came and filled the apostles and they began to speak in other languages (Acts 2:4). In his subsequent explanation of this strange event, Luke has Peter quote from the Old Testament prophet Joel, explaining that in the last days, the Holy Spirit will be poured out on all people with attendant signs that include prophecy, dreams and visions (Acts 2:17).
 Because of these signs (and many others) people will call upon the name of the Lord, which will in turn lead to their salvation (Acts 2:21). Thus, Luke sets the stage for dreams and visions to mark the movement of the Holy Spirit in the work of the early Church, affirming God’s agency behind her mission as it backs Jesus’ ministry in the first half of Luke’s story (Ehrman 147).
Sometimes visions mark divine approval or disapproval of the actions of characters within the story. For example, Luke uses a vision of an open heaven (Acts 7:56) as confirmation of Stephen’s divine mandate when speaking before the Sanhedrin. Though the religious authorities have found him guilty of blasphemy and are stoning him, the vision of the open heaven reveals to the reader that Stephen has properly understood the gospel of Jesus Christ and has obediently followed his commission. This scene can be contrasted with Saul’s vision on the Damascus road in which God chastises him for standing against the spread of the Gospel (Acts 9:4-5; 22:6-7, 19; 26:14-15).  In these accounts Luke asserts that the risen Jesus Christ directly challenges Saul’s zeal for the persecution of the nascent Church, siding with those who spread the gospel throughout the synagogues of the Jews.
Several visions within Acts provide direction or encourage boldness. After the Damascus Road confrontation, the Lord Jesus speaks to Ananias and directs him to go to minister to Saul, reassuring the disciple that he will not be harmed, but will be carrying out a part of God’s plan (Acts 9:10-17). Later, Paul too is encouraged by a vision in the face of danger. Luke tells us that when Paul comes into conflict with the local synagogue in Corinth, Jesus directs Paul not to fear, but to speak the gospel. As a result Paul remains in Corinth for 18 months, teaching among the Gentiles (Acts 18:1-11). In both of these instances Luke’s use of visions bolsters the confidence of those in potential harm’s way to be bold in carrying out direct instructions from Jesus.
Similarly, dreams and visions within Acts direct characters to sweeping actions that they would not otherwise undertake. It is the vision of the blanket filled with unclean animals coupled with a command to accompany soldiers at the door that prompts Peter to share the gospel message with the Gentile Cornelius. Before the intervention of the Holy Spirit, Peter had believed that the good news was only for the Jews; however, Luke writes that Peter’s framework has expanded to include Gentiles who fear God and do what is right within the number of those whom God finds acceptable (Acts 10:34-35). Without the vision in which he hears that he is not to call unclean what God has made clean (Acts 10:15), there would be no impetus for Peter to proclaim the gospel outside the confines of his cultural world. Indeed, Peter falls back on his divine mandate when questioned by his fellow Jews regarding his preaching among the Gentiles (Acts 11:5).
Following this model, Luke also introduces a dream in which a man pleads with Paul to undertake a missionary journey to Macedonia (Acts 16:9-10). As a result, Paul immediately leaves Troas and travels to Philippi. Without the vision there is no particular reason why Paul might travel along this route; however, Luke’s inclusion of the dream provides a reason for going and a confirmation of God’s role in the founding of the Philippian church.
Thus, through his use of dreams and visions, Luke affirms the mandate of the early Church as a continuation of the mandate of Jesus’ own ministry. They are part of the larger theme of signs and wonders that accompany the spread of the gospel, providing visible evidence of the ongoing work of the Holy Spirit in the world (O’Day 395).  Demons are cast out, people are set free of various afflictions. And in moments of both tumult and quiet, God’s spirit intervenes directly within the narrative to provide comfort, to legitimate claims and to direct the actions of the disciples.
Erhman, Bart D. The New Testament: a historical introduction to the early Christian writings. 4th Edition. New York: Oxford University Press, 2008.
O’Day, Gale R. "Acts." Women’s Bible Commentary. Ed. Carol A. Newsom and Sharon H. Ringe. Expanded Edition. Louisville: Westminster John Knox, 1998. 394-402.