Why is that even a feature of browsers? On my desktop I can zoom whenever, wherever I want. Why should it be any different on mobile...

Edit: In mobile safari I have a bunch of bookmarklets (javascript bookmarks) for enabling zoom, and to zoom the way desktop browsers do (increase text size and reflow text).

It's for websites that want to mimic apps.

Mimic apps that break on screen resolutions other than the tested ones?