at least a couple of reasons I can think of:

• incentives: with your own coin you can design incentives to stimulate behaviors in your target community
• bootstrapping: with emissions of your own coin you have a chance to bootstrap initial users without paying a lot for customer acquisition, e.g. I just signed up for Steemit and I got equivalent of $0.70 value on my account to get me interested to play (you can say to condition me to use it and to earn it), something that would be expensive to award in ETH. Second Life used to give a stipend in Linden Dollars to get you started in-world (not sure about now, as have not visited it in a while).