Why do people do this? Some things are better left unsaid. If you got coins from selling a mountain of weed it is better to not say it. If you need to unload some coins cheap because you stole them it is better to not say it. If you are sending coins to the Silk Road it is better to not say it. Most certainly if you are planning of evading taxes it is better to not say it.
Why? Why do people do this? What possible good could come from divulging the reason for your transaction? Does nobody care about financial privacy?
I'm not doing that, it was just a joke ...