Power. As good as the Hugo 2 is at driving most headphones it's output is limited. Demanding headphones like the Susvara, HE-6/se, and Abyss Phi's sound their best when driven with more power. Using the Hugo 2 as my DAC feeding my Pro iCan I find that it sounds sonically superior than using the Hugo 2 alone with those headphones. My Utopia's, Z1R's and LCD-2CB's sound amazing out of the Hugo 2 alone. So depending on what headphones you have in your collection you can choose to live with the Hugo 2 alone. For me, I love having the power and feature set of the Pro iCan on hand.
This is immensely helpful! Given the stuff I own, I think only my HD800S would benefit sufficiently to justify the expense (and for those occasions, I have my ifi Micro BL)...
Thank you for this information.