there are so many new projects around. people gets to choose which platform that is favorable to them and trade on it. the disadvantage is that it might be a scam exchange or it could be hacked if security is flawed. the benefits of having many exchanges is the projects should have varieties to choose from.
Yes, more exchanges mean a greater variety to chose from, but it also means a decreased liquidity and trading volume, since that particular coin can be traded in many exchanges and so everyone has less trading operations...