When the Lord comes down in human form, sometimes it is essential to perform some miracles for the sake of certain deserving devotees. For the highest devotee, miracles are not necessary. They believe in the Lord even without miracles.
For the lowest devotee or a person who is not a devotee and one who is not likely to turn to God anyway, miracles are of no use. Miracles are necessary only for devotees who are in between these two levels; those who mostly have faith but sometimes have doubts.
Krsna did not show His Divine Cosmic Form (Visvarupam) to Radha, who was the highest devotee. Duryodhana was the other extreme. Even though the Lord showed Him the Divine Cosmic Form, he did not change. He continued with his evil ways and fought the war against the Lord.
The miracle of showing the Cosmic Form was useful only in the case of Arjuna, who was generally very devoted to the Lord, but at that moment had some doubts.
The human incarnation of the Lord performs miracles to prove that God exists in that particular human body. However superpowers and miracles are not the essential identifying characteristics of God. They are only associated characteristics.
As mentioned earlier, superpowers can be given to any person, good or evil, if he does adequate penance and asks the Lord for the boon of those superpowers. Therefore if someone has certain superpowers, it does not necessarily mean that he is a human incarnation of God.
The crown of a king is an associated sign of the king; it can be given to anybody. Suppose a king is very generous and gives his crown in donation to a beggar. The beggar cannot be called the king just because he possesses the crown. However the physical form and features of that king are his essential identifying signs. They cannot be donated to anyone. Where they are the king always is.
When devotees or even demons perform rigorous penance for getting superpowers, the Lord grants them the powers. Suppose a child pesters his father or a golden ring. The father, in frustration gives it to the child. Some thugs see the ring on the child and kill him to get the ring.
The Lord is the Father of all souls in creation as per the Bhagavad Gita (Aham bija pradah pita) When His children act stubborn and insist on getting those powers, the Lord reluctantly grants them their wishes. Unfortunately, they are unable to handle the powers and are destroyed by them.
When demons or ordinary persons, who have superpowers, perform miracles for their selfish ends, a lot of people are fooled into thinking that those persons are incarnations of God. However, great sages do not get fooled easily. They do not accept a person as an incarnation of God just by seeing his miracles.
Real devotees of the Lord too are not impressed by such miracles. Sita did not accept the evil Ravana in spite of his miracles. On the other hand, Lord Rama, who was an incarnation of the Lord, never performed miracles in public. In his life time He performed only one miracle in the presence of Sage Visvamitra and His brother, Laksmana. Both of them were deserving devotees. The miracle that He did was that He touched a certain stone by his foot and that stone became a lady called Ahalya, right before their eyes.