10:1
And he called to him his twelve disciples and gave them authority over unclean spirits, to cast them out, and to heal every disease and every affliction.
gave them authority
The Lord invests the apostles with the authority to perform miracles with a purpose. Miracles are the way God attests to the words of a messenger.
In John 9, the man born blind whom Jesus granted sight understands well that the miracle must mean Jesus has God's approval and endorsement.
John 9:30-33
The man answered, "Why, this is an amazing thing! You do not know where he comes from, and yet he opened my eyes. We know that God does not listen to sinners, but if anyone is a worshiper of God and does his will, God listens to him. Never since the world began has it been heard that anyone opened the eyes of a man born blind. If this man were not from God, he could do nothing."
When the apostles later meet strong resistance to the gospel, God provides support--for His word--by means of miracles.
Acts 14:1-3
Now at Iconium they entered together into the Jewish synagogue and spoke in such a way that a great number of both Jews and Greeks believed. But the unbelieving Jews stirred up the Gentiles and poisoned their minds against the brothers. So they remained for a long time, speaking boldly for the Lord, who bore witness to the word of his grace, granting signs and wonders to be done by their hands.
We rely on the testimony of the apostles because God showed they were His by means of the miracles accomplished among the people.
Hebrews 2:3-4
[Salvation] was declared at first by the Lord, and it was attested to us by those who heard, while God also bore witness by signs and wonders and various miracles and by gifts of the Holy Spirit distributed according to his will.
Finally, Jesus not only grants miracles to be done, but He also places them into proper perspective.
Luke 10:17-20
The seventy-two returned with joy, saying, "Lord, even the demons are subject to us in your name!" And he said to them, "I saw Satan fall like lightning from heaven. Behold, I have given you authority to tread on serpents and scorpions, and over all the power of the enemy, and nothing shall hurt you. Nevertheless, do not rejoice in this, that the spirits are subject to you, but rejoice that your names are written in heaven."