Over the years this tradition of angels accompanying St. Nicholas grew as devotion to St. Nicholas was solidified.
For example, in Austria both “angels” and “devils” would accompany St. Nicholas as he visited houses on his feast day. A periodical from 1897 sheds some light on this European custom.
The character of St. Nicholas in Austria is usually taken by a young man versed in the church catechism. He is arrayed
in a long, white robe with a silken scarf and a miter and scepter. He sometimes has with him two angels, dressed somewhat like choir-boys, each of whom carries a basket or bag, and along in the background follow a troop of devils … In the twilight of the fifth of December the bishop makes his rounds to the various houses where the children are collected in parties. He enters with the angels, while the devils wait outside. A great silence falls upon the assembled company, and the children are called up and examined religiously. This is carried out with great seriousness. If the trial is passed successfully, the angels step forward and give the child gifts and nuts and cakes … When the inquisition is over, the devils are allowed to enter and frighten the children … and amuse them with their strange dances and antics … After St. Nicholas has departed the children go to their homes, with the expectation that St. Nicholas will visit each house separately and be more generous and bring them more gifts. So, after saying their prayers with more than usual earnestness, they put baskets and dishes on the window-sills and go to bed.
The way this is celebrated varies according to the region in Europe, and has even grown to incorporate the modern depictions of St. Nicholas. Angels are often explained as the primary helpers who assist St. Nicholas in visiting all the children in the world.
Suffice to say, this tradition has much truth behind it, as Catholics believe angels are real and do help us with our charitable deeds.