Because if you add any halfway decent source, even a phone (digital out), the sound quality is untouchable at this price. In review after review after review, from Headfonia, to Head-Fi, to Stereophile, no one can find much of anything bad to say about this product. I have its bigger brother, the Hugo. It travels with me everywhere with my TH-X00 in a road case.
In fact, I liked the sound so much, I bought the TT for my desktop rig.
Using a computer or an AK source-- native DSD files through the Hugo, Mojo, and Hugo TT are overwhelming.
If you want to improve your Mojo from there, you can add a Cavalli or ALO portable amp on the back end and punch up the quality a notch higher. But the sound is very good out of this tiny little box, and the internal amp has plenty of power unless you're trying to drive an HE-6 or Abyss.
There are better stand-alone amps, but if you're amping a mediocre source, you get mediocre sound.
For the TH-X00 the Mojo and Hugo are magic combinations. You don't have to take my word for it, do a quick Google and read what people are saying about the Mojo.
Watch eBay, too, and Massdrop. Deals pop up from time to time.