That is something very useful to people who find characters to be often too small on the screen while browsing the web (a lot of people). Strangely, almost nobody knows that this feature exist! No need for glasses anymore..
With the menus
- Click on the wrench
- Click on the + to enlarge, or the - to reduce the zoom.
With a keyboard shortcut
- Use CTRL and + to enlarge, CTRL and - to reduce the zoom.