the benefit is that, (a) if you need a few miles to top off a redemption, AND (b) you don’t have a better way of spending the credit, this is an option. It is also cheaper than buying miles through the portal. The other benefit of this is that you can pick any flight you want and get the closest flight multiplier you need. So, if you just need, say 1800 miles, you can play around until you get the right flight that will give you around that in miles multiplier...

The only caveat make sure you book a flight far out and cancel within the 24 hours, but if it is within a week of the trip I think you cannot cancel so that would be an epic fail.