18 they arrested the apostles and put them in the public prison. 19 But during the night an angel of the Lord opened the prison doors and brought them out, and said, 20 “Go and stand in the temple and speak to the people all the words of this Life.” 21 And when they heard this, they entered the temple at daybreak and began to teach.
Now when the high priest came, and those who were with him, they called together the council, all the senate of the people of Israel, and sent to the prison to have them brought. 22 But when the officers came, they did not find them in the prison, so they returned and reported, 23 “We found the prison securely locked and the guards standing at the doors, but when we opened them we found no one inside.” 24 Now when the captain of the temple and the chief priests heard these words, they were greatly perplexed about them, wondering what this would come to. 25 And someone came and told them, “Look! The men whom you put in prison are standing in the temple and teaching the people.”
The miracle of the release from prison is to show that nothing can stop the gospel spreading. It is not primarily for the apostles benefit, but to show the leaders and the people that God's message can not be locked up - he can simply open the doors. The apostles understood this as they went straight back to preaching, not into hiding.