Ok I’ve rechecked it and it seems I was wrong.
On Android it turns on this option:
Sets whether the WebView should use its built-in zoom mechanisms. The built-in zoom mechanisms comprise on-screen zoom controls, which are displayed over the WebView’s content, and the use of a pinch gesture to control zooming. Whether or not these on-screen controls are displayed can be set with setDisplayZoomControls(boolean). The default is false.
The built-in mechanisms are the only currently supported zoom mechanisms, so it is recommended that this setting is always enabled.
If it does not cause any effect might it’s conneted to your website – e.g.
<meta name='viewport' /> is missing?