overlordrorphysical size and the 2.4A usb output.
though i can't say it's actually attractive at the price, it is a viable option for portable rasPi3 projects since for the rasPi3 they recommend 2.5A and the instructions say you should have at minimum 2A (or was it 2.1?)
i bought a 20,000 mah battery to use with my Pi3 though it's large and heavy but for my project those two things didn't matter.
https://www.amazon.com/gp/product/BO1DP4IJWS