Spinning with rolags vs "fauxlags"
First things first: as far as I have been able to determine, there is no functional difference between rolags and "fauxlags." It's just one of those things where spinners insist that you get the jargon right.
A rolag is a little burrito of fiber which is created by using hand cards. You card the fiber, and then carefully roll it off the hand cards into a tube. It's like a very small batt, about the size of a hot dog, with light, airy, fluffy fibers.
By comparison, a fauxlag is one of those that you make by hand off a commercially prepped fiber. Fauxlags work best from a combed top, which is the one where all the fibers have been combed into alignment. (Versus carded roving, where the fibers are all jumbled together.) (I know, I'm terrible about remembering the difference between them, myself.)
Why would you choose rolags, or make fauxlags? Two reasons: color control and drafting style.
If you buy a dyed batch of combed top, you may want to go your own way with how the color repeats stack up. You might want the repeats to be shorter, longer, or in a different order than they appear in the combed top. Making fauxlags lets you rearrange the colors and repeats to your heart's content.
The other benefit to spinning with fauxlags and rolags is that they are ideal for long draw drafting. The loose, fluffy fibers really "catch" the twist nicely and pull it out easily and evenly. Spinning from rolags gives you a woolen yarn, which is light and fluffy and lofty (although not as strong as a worsted-spun yarn). If you want to create a woolen yarn out of a combed top, then fluffing it out into fauxlags is definitely the way to go.
Image courtesy Flickr/Hedgehog Fibers