Reason number one:For pump&dump purposes.
Reason number two:Exit scam ICO(which is kinda like the first one,but with pump&dump they could pump the coin again and gain aditional profits).
Those are the real reasons.The fake reasons include:instant and cheaper transactions,easy and profitable mining,technology innovation that will beat the crypto competition.
I'm sure that majority of cryptocurrencies are like that, they are created with a purpose of making their creators rich and will die eventually because there was never real goal of developing them.
However, some developers start their own cryptocurrencies because the field is still very new and they think that they are smarter than others and can create a coin better than all other coins or a coin with unique features. Take Vitalik Buterin as an example, he wanted to become Bitcoin Core dev but was rejected, so he started his own project.