God works miracles to build faith in His Word. The absence of miracles will often cause people to doubt God's Word. This was why Moses asked God to be with him to perform miracles. This is why we are required to do the same things Jesus did and take up the work He left off. WILL Miracles are performed to deliver people in danger, to perform judgments upon those who interfere with His work, to calm storms, to cast out devils, to bind up broken hearts, to deliver the captives, and to deliver people from natural stubbornness and rebellion so they may receive the Holy Ghost.
Miracles cause people to be saved.
Elijah had power over rain. The bears ate the children who mocked Elisha. The men who came to arrest Elijah were struck dead. Naaman's leprosy possessed Gehazi. Moses had power over rain, hail, and frogs.