When The Lord sent his disciples out to preach the Gospel, he gave them the power to perform miracles which would confirm their teaching.  St. Thomas Aquinas, in his commentary on the Gospel of St. Matthew, addressed a very good question about miracle-working:

"Why does the Church not do miracles now?  The answer is that miracles are done to prove the Faith, but now the Faith is approved.  And just as someone performing a demonstration in order to prove a conclusion does not need to do so again, so it is in this case.  Now, the greatest miracle is the conversion of the world, and so it is not necessary for other miracles to be made.  And just as some miracles are physical, so there are spiritual miracles that are done every day because the spiritually sick are healed.  The sick are those who have committed sin and so are in sin.  Romans 14, 1: 'Now, take unto you him who is weak in faith', and these are healed by The Lord.  Those who consent to sin are dead because they are separated from The Lord, and these are revived by The Lord.  Ephesians 5, 14: 'Arise, you who sleep, and rise from the dead.'  Likewise, the lepers are cleansed.  Those who are infected by something foreign are called 'lepers', and the spot [lepra] is a contagious sickness.  These are sometimes cured.  In 4 Kings 5, it is written that the leprosy of Naaman stuck to [his servant] Giezi.  Also, demons are cast out.  The demons are those whose sin passes into effect.  As it is written of them: 'They [the demons] rejoice when they [sinners] act badly, and they exult in their worst deeds.'  And, as it is written of Judas, in John 13, 27: 'Satan went into him.'  Sometimes these are cured."