If you read their whitepapers you can see what the new coins are promising.
For example, I have seen a totally unnecessary one just recently, there was this guy who is running a marketing agency (well he is not running an agency, he is just one person doing freelance) and he wanted to start a new token and get it going because he believed he could get advertisers and people who need marketing together and just build a platform where everyone can either work for marketing and make money or spend that token to get their marketing needs met. Now I am not a big marketing genius but you can build a platform like that in bitcoin or any other coin we have right now.
Why would people use a new coin. There are a lot of people who keep on thinking they can make a new coin with a smart idea behind it but no one replies to "why wouldn't they use bitcoin instead" properly.