These are of two kinds: their functions in the physical world, and the spiritual development of man.
In the physical world, the working of nature is governed by laws, as science has shown by discovering many of these laws. Islam teaches that these laws have been devised by God, and the angels - the obeying functionaries of God - put them into action.
On the spiritual side, the angels communicate God's revelation to the Prophets and other righteous ones, bring comfort and strength to the hearts of true believers, and inspire noble thoughts in the minds of all persons. They do this, of course, by acting through the spiritual senses of human beings, not their physical senses like the eyes or ears.