What is the advantage of a market-specific cryptocurrency? Reading this article, http://nakamotoinstitute.org/mempool/the-coming-demise-of-altcoins/, I wonder why there would be a separate coin for each market, like a coin just for buying bananas, marijuana, disk space or photh-sharing instead of using a general purpose coin for those things?

I'm not asking why coins are technically different; having different hash-functions, mining hardware, proof of work vs. stake, virtual machines, privacy, how the coin is governed, etc.

I am asking why there are coins for each particular purpose.  I think it would be better to only use one coin for everything because there would be less middle-man in the exchange from BTN to STORJ and vice versa therefore lowering fees.
Why is there more than one fast-food service? Why is there more than one internet browser? The competitive altcoin economy encourages other coins to surge with better implementations. Bitcoin has very high transaction fees as of early 2018, this is why a fork came off as Bitcoin Cash a few months ago, but Bitcoin Cash is not the only coin fighting for low fees. We have seen RaiBlocks (No fees) and many other altcoins surge recently due to low fees. Some coins are created not only to solve a problem, but to simply be better than the current coin. NAV wants to surpass Monero, by not only being a privacy coin, but also allowing anonymous Dapps and more.

A lot of coins have lucrative companies behind them, the answer to this question is not really different from the one of the question "Why is there more than one company in sector X"? People want to make money, so they will innovate and try to outperform the others with a better product.

Sometimes, companies will issue a token usually from an ICO to raise money on their platform. There's usually little reason for the purpose of their custom token/coin other than raising capital. A normal company would have to attract angel investors which is a lot harder than creating a coin and selling it while cryptocurrency is a hot asset. Doing an ICO also does not require to lose a share in the company. This is why we are seeing a massive influx in new coins.