ZIX acts as a deposit within the smart contract to prevent fraud or double spending.
The ZIX token serves several purposes without which our protocol could not function. ZIX acts as a login to the system, allowing users to buy and sell corporate currencies. ZIX can of course be used as a payment as well, but it can also be used to reserve various amounts of corporate currencies that may be in higher demand, so you can be sure you can get ahold of it (for instance, if there's some crazy discount like 20% on Amazon gift cards, you might wanna reserve that.)
For related Medium Article, please see:
https://medium.com/the-zeex-protocol/why-does-zeex-need-zix-8ad243aaae60