Why? Because this is the only way to find all errors your users see but you don't. Only thing left that could fail without you knowing about it is the service or framework itself so let's hope that's well tested or... errorception!
I guess it should possible to log something like "okay, I'm loaded and ready to catch them errors" to make sure everything works as expected but I degress.
You can also read something like http://blog.meldium.com/home/2013/9/30/so-youre-thinking-of-tracking-your-js-errors about JS error tracking.
Below you will find some resources I collected during a very not extensive Google search, I might add more later.