a) You need to know it anyway.
b) Node.js is awesome and works the way web servers ought to work.
c) JavaScript, if you avoid the obvious warts, is a really nice language. Every other language I've learned either has as many warts as JavaScript or isn't practical.
Having picked JavaScript, it turns out to be amazingly powerful. Need to automate content production? Adobe's stuff is all scriptable via JavaScript. Want to write a 3d game? Unity uses a dialect of JavaScript.