The use of Google API can add more functionality to your site. There are a lot of Google API's. You can find more info on http://code.google.com Example: if you use the Google maps API you can embed google maps in your site.
There are so many APIs available. May be better question would be: "I want X can Google API do this?".
Google API allows developers to add functionalities to their websites using the data Google houses. You can make Trulia-type google maps with the google maps api or you can make custom maps outside of Google Maps' MyMaps You can basically develop whatever app or code required to add systems/programs/apps to the pages of your site or you can even make a web app which uses the google APIs and you can make money off of it -- i.e. WordTracker
so that's for software developers not for web developers. because if i m a blogger or content writer i dont need that api..coz it will be very hard to implement it.
benefit of Google API is that you can to add functionality to your websites from the data Google houses........