Because some new Coins can fill a niche of a particular market in some ways or can offer improvements in which larger Coins cannot. Most every Coin is a clone of another Coin including the mainstream Coins, often many of these are much better than the mainstream Coins in certain ways and offer the chance to try new things. The majority of them are pump and dumps though so if you are interested in any particular one, you should research and see how development is going after a couple months and if it's got activity and developer working on it then by all means take a position in them.
The key is to do your own research. I agree with the quote above. There are two types of coins, those who hope to replace bitcoin with newer technology and those who fill a niche.
I am not a day trader but a longterm investor, and frankly there are so many coins offering new technology in hopes of competing with Bitcoin, I am not smart enough to pick the winner of that race, if there will ever be one
So I hold longterm positions and accumulate more when the price swings. My top picks are AppleByte, which supports the artist community and Gulden, which is gaining a foothold in the Netherlands. I also hold Doge from way back, as I like their niche in social media tipping.