As Tom says, the view of angle (and the area covered by the "largest" third of the co-ordinate system) will allow you to draw on any of the 3 default planes.
You can also use one of the standard views (top, left-right or front-back) to draw on the blue, red or the green axis (just push the button and it gets you there then push the "back" button in the camera menu bar to get back where you were previously).
However this is mostly a handy way when there isnt too much geometry around to which the circle tool tends to snap easily.
And also it is a kind of a matter of workflow - whatever you get used to (first). Eventually you can even rotate a circle into a desired alighnment.