I have never seen a circular crochet hook. I imagine that it could be used for circular crocheting in the same way as with knitting. But probably the reason for this type of hook is to hold a great many stitches in order to make blanket sized material specifically a "40" wide panel.

You first crochet a chain as long as you need, then you proceed to put loops on your needle. The second row is worked off by first going through the first length-wide loop and thereafter through two loops. The third row is done by putting on loops by going through the length-wide stitches until you finish at the end of the row. Repeat the second and third rows for the length you need.