Server-side languages, yes. There are lots of those, so each one doesn't impact many. But there's only one client-side language on the web, so it impacts everyone.
Yes. JavaScript is lingua franca of the Web, so it may be important to have a particular functionality implemented in JavaScript.
This way, if you, say, want to take a part of the program and use it elsewhere, you have the advantage of the same language. If you want to ask a question about the program, you can count on other people knowing the language. If you want to illustrate something using a program, you have a common ground here.
