The name itself signifies. The original Hebrew meant "he who is like God;" in Christian traditon, that description morphed into a question: "Who is like God?" which in turn was held to be Michael's battle cry as he and his host ejected Satan and his host from heaven for a mass rebellion motivated by Satan's claim to God's throne. Thus Jesus said: "I saw Satan fall from heaven like lightning" (Luke 10:18). Now I know there are plenty of biblical scholars who explain that away, in context, as nothing but Jesus' approbation of the work of "the seventy-two" in rolling back evil. It was that, to be sure; but frankly, I don't buy the reductionistic 'nothing but' and have never been able to. With Tradition, I believe that Jesus was also describing what he witnessed in illo tempore, as God the Son, before his incarnation as a man. His copious talk of and conflict with Satan in the Gospels strongly suggest that the two go way, way back: further back than we can possibly imagine. God appears to have given Michael the place of honor once held by Lucifer, now Satan. As such, Michael is commander-in-chief of the angelic hosts and champion of the Church Militant on earth.
Many other roles, revealed in apparitions and miracles, have also been attributed to him. Google him, read about him, and learn. This saint is incalculably important, as are angels generally in the "economy" of salvation.
Some people are put off by the fact that he and the other archangels are called 'saint'. After all, Rome hasn't vetted and canonized any such being! That betrays a misunderstanding of the word. The lower-case 'saint' comes indirectly from the NT Greek hagios, meaning 'holy one'; the Latin for that is sanctus, the root of the Old French and Norman saint. Any "holy one" is a saint, even if the pope doesn't know it and say it. All the good angels are saints; they dwell in God's presence and do his bidding. Indeed, the very term 'angel' comes from the Greek for 'messenger'; in the Bible, that role is the most common one attributed to angels for the benefit of human beings.
I am devoted to St. Michael as protector, guide, and hopefully as healer. I concluded a novena to him today for those closest to my heart.