It's useful for a lot of people to have a name for it would be my immediate answer.Can anyone explain what is the difference between a) being asexual and b) being at the far left of the spectrum of sexual libido, where 0 is zero libido and 10 is permanently horny (obviously most people fluctuate throughout life anyway).
Why does it need a special name?
It can be more than lack of libido. Some asexual people don't experience sexual attraction at all, some have no interest in a romantic relationship. Not having sexual desire or even attraction can be distressing. As mentioned in another post, society is completely driven by sex. So you're led to believe something is wrong with you for not feeling that way.
Just an anecdote but a friend of mine is asexual and she put herself in a lot of unsafe situations with me when she was younger, trying to sleep with as many as possible to try and find some sort of sexual desire or attraction. It never happened but had she been aware of asexuality at the time, things would have turned out very differently.