The idea of this feature is to dinamically magnify the screen which means that you can move your mouse and type characters from your keyboard and the screen will update itself in the meantime.
The most obvious usage is just to see some texts better like this
In YouTube there are some videos which are displayed on only half of the screen. Even on fullscreen the subtitles are small. Example
You can magnify the screen and watch it like this
And other use cases like this.
You have also shortcuts
Zoom in the Screen
Zoom out the Screen