it makes the page less sluggish. it's impossible to disable double tap zoom without disabling pinch zoom, and while double tap is enabled, browsers have to insert a tap delay. this is because every single tsp could be the beginning of a double tap, so the browser waits until 300ms after a single tap to respond to it. this delay isn't perceptible as a delay, but it does make the entire page feel less responsive. disabling zooming disables the delay and makes the page feel quick again.
also disabling zooming makes it feel more like a native app, which some people like.
It's easy enough to avoid the 300ms delay without breaking zoom. There are dozens of tiny libraries that fix this problem by intercepting touchdown events and immediately firing a click if there isn't a touchmove within a very short interval. So, double tap doesn't work, but pinch does and you get "tap" events with very little delay.
