The property in CSS is used to magnifying the size of the content whether it is plain text, images, graphical elements etc. Magnifying may be for increasing the size of the element or decreasing the size of the element, it is completed depends on user requirement. In earlier versions there is no zoom attribute for zoom functionality, user has write their own logic to implement zoom functionality. This predefined "zoom" attribute reduce lot of code to write.
Note: This "zoom" attribute may not allowed by all the browsers due to compatibility issue. Common supported browsers are Internet Explorer, Safari, and Chrome.
Real Time Example: Let suppose some xyz company employees some of them are having near sight so they can't see things which are small, so they can't see browser content if it is normal font size so we have to zoom in the size. If some of them are having far sight so they can't see things which are big, so they can't see browser content if it is normal font size so we have to zoom out the size.
Note: This "zoom" attribute may not allowed by all the browsers due to compatibility issue. Common supported browsers are Internet Explorer, Safari, and Chrome.