They were used in loooots and lots of paintings, mainly to depict saints / "blessed ones" but in general, it is a great way to separate the subject from their background.
It also looks really cool. One of my favourite paintings that uses this (but in the more literal sense) is by Paul Delaroche, and is called "The Young Martyr"
I believe it is inspired from a bible story, but its such a beautiful and moving painting, with the figures stumbling across her in the background, and the halo being a light source for the floating figure.