At it's core, ZRX is a governance token--you hold to have a say in protocol upgrades. dApps and relayers will want to buy and hold some so they can have a say in protocol upgrades. With over 20 dApps and relayers using it as a core part of their functionality and revenue models, you can imagine the potential impact there in the long term.
It is also the token used to collect fees for Relayers. I.e., if an exchange uses 0x and collects fees, ZRX is the token they have to collect fees with. Although some people speculate that most relayers will immediately resell almost all of their ZRX to Eth and fiat (and indeed they will sell at least some of to operate), what many don't realize is how quickly these relayers could become whales of ZRX and influence the price if they want to. This isn't a bad thing for the holder--they would never want to influence too dramaticly as that would make the charging rate for fees in their exchanges problematic, but by holding and controlling for a slow increase in the price over time (which they are also incentivized to hold for governance anyway), and strategically releasing in batches over time they can maximize their profits pretty easily. #cryptocurrency