The Buff doesn't necessarily do anything different, just better IMO. While the standard bandana will absorb water quicker, the Buff dries in about half the time. I have soaked one of each and dried them side-by-side. I did not, however, measure the amount of water absorbed by each; I just ran water in the sink to soak in.
That said, I use the Buff for wearing and use a trimmed MSR ultralite towel (around .7oz) for drying hands, wiping condensation, etc. I haven't seen one of the extra large bandanas since I was a kid, and the standard size (I think they are 22" square?) don't stay on my funky shaped melon very well. They also don,t work well for a neck gaitor. Besides I used to carry two bandanas anyway, that way I wouldn't have to use my headwear for a condensation rag. Might as well have two items that are better for their respective tasks, and weigh the same. Also, it replaces a warm hat for 2.5 seasons because my warm jackets have hoods.
I understand what you mean about having a hard time justifying the cost to yourself. I couldn't when I first saw it. Then I saw an ACU version made by SpecOps in a PX (it matches the pattern of the new Army Combat Uniform). I bought it with the intent of using as a dust mask, but it was so versatile that I still carry it with me anytime I'm in uniform, even in warm weather, in garrison.