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.