Because WordPress is built using a number of languages none of which are compiled, it makes it completely possible to make things happen within your theme, plugin, or extension by circumventing the native APIs.
This means that if you wanted to, say, introduce some type of element on one of the dashboard screens or you wanted to introduce functionality into one of your templates that didn’t previously exist, there’s a strong chance that you’d be able to do so simply by “brute force.”
And by that, I mean that you’d be able to make something happen – and probably work correctly – without using the native set of APIs that are available.
But when you’re faced with that situation, I highly recommend taking a step back and determining if you’re approaching the problem in the best way possible given your set of constraints.