"Then fear came upon every soul, and many wonders and signs were done through the apostles."
Acts 2:43
The Holy Spirit would come upon the disciples, [not just the Twelve], but all of them.
This was the Promise of the Father.
And they would be endued with power.
They had received authority to preach, heal and cast out demons just before Jesus ascended to heaven.
Now they would receive power when the Holy Spirit came upon them.
This is the same power that Jesus healed the sick and cast out demons.
The purpose of this Holy Spirit power is to be a witness to Jesus.
A witness is one who presents evidence or proof.
Witnesses to Jesus present proof that He is truly the Anointed One, the Messiah.
We do this by preaching what Jesus preached and doing what Jesus did, which includes healing and casting out demons.
Luke's use of signs and wonders relates back to Joel's prophecy, which was cited in Acts 2:19 "I will show wonders in heaven above, and signs in the earth beneath."
In the book of Acts, signs and wonders generally refers to healing and casting out demons.