Indeed. Some of the tin-hat brigade like to use them because they are sure that the government have nothing better to do with their time than log in to the "GPS SPY CHIP" that the colaborating manufactuers have installed in their car and so know where they are all the time.
Now, to be fair, the DS does have such a spy chip. You can log in to it from your phone at any time and ask your car where it is and where it's been.
But unless you're a terrorist or a bank robber, I very much doubt HMG give a hoot.
So - you need one of these if you are a terrorist, or a bank robber, or, indeed, if you just stole someone's DS and you don't want the owner being able to find out where it is from their phone app.
Number of legit reasons to have one I can think of - nil.
Oh, you might think this would mean the government would make them illegal. Guess what? They already are - they transmit on frequencies on which you are not allowed to transmit without the appropriate licence so illegal to use. Not illegal to sell or own, sadly, even so I was surprised by just how many sites are sellign them in the UK.