New languages are useful when they introduce vastly different paradigms: not necessarily something new in itself, but something that they bring as a first class citizen of their ecosystem.
Take Ruby vs Python. Different syntax (both are either a marvel of software engineering masturbation, or the worse thing since Global Warming, depending on who you ask), but aside for that, conceptually, they're very similar. They can be used as scripting language, they have dynamic features, they're fairly high level. Their ecosystem is also similar, from the way packages are maintained, to what they're used for.
The primary differences as to why they're used, like python's math libraries (which have been facilitated by some of the language's particularities, but still), or Rails, could have happened anywhere. One of those 2 languages is largely redundant, but the category of languages they belong to, is not.
C/C++/Objective C/Go/Whatever could also be bucketed together, but the category is useful: higher level languages that have native compilation as one of their core, and the ecosystem revolving around them reflects that.
Java/C#/Scala as the languages with more sophisticated underlying runtimes/garbage collectors, etc.
JavaScript on Node.js, while it may look a bit like Ruby because of the ecosystem, has interesting/useful performance characteristics, and is useful in itself because it allows one to use the same language they are semi-forced to use in web apps, everywhere else. There's value to that.
The list goes on, but not much. You only really need 1 language per "category", and everything else is redundant, with a few outlier languages that bring some values on their own. Adding another yet another functional JVM language is useless. Making another loosely typed yet OOP scripting language with a package manager and built in REPL for web development would be fucking pointless. Another natively compiled language to compete with C++, when C++ 2014 has most of the modern features, would be another waste of time unless it does something REALLY different, because you then need to interop or rewrite all of the C++ ecosystem.
The Ruby/Python/Node/Blah category seems to be the biggest offender by far. There's so many fucking variations at this point, its ridiculous. All of the JavaScript replacements with transpilers are also just syntax masturbation and bring nothing of value. AtScript vs TypeScript vs CoffeeScript...lol.