I was thinking the same thing!
After a quick poll of the office, no one could come up with a legitmate legal reason why you would use a Tumbler Service.
The closest we got was that perhaps for the funding of Spies, etc. to hide where the money comes from, but even that's stupid. It would be much easier and more secure to create a new anonymous wallet, and transfer the funds from there. the use of a Tumbler Service on the funds being sent would probably act as a red flag in the first place. So you know it would be a dumb idea anyway.
Anyone got a legit reason for these services to exist? Apart from making their owners a little bit richer at the expense of regular crypto users (also known as Victims or Potential Victims!).