Is the viewport meta tag really necessary?

I've created a few responsive sites but am rather new to responsive site development. In my CSS 99% of my values are in ems or percentages. I am using media queries (both max-width and max-device-width) to make layout changes. I have not included a viewport meta tag and it works perfectly on iOS, a number of Android phones and tablets that I tested on, and all desktop browsers.

Adding a meta tag breaks my site. Am I doing something wrong, or doing something right so that I don't need to include it? I'm confused as to why it seems to be a best practice, as it's breaking my stuff.

Am I missing something?