Visual effects: This helps in decoration and nowadays Advertising.
Games: Interactive games are so much in demand online nowadays.
Banners: customized banners can help in delivering messages and warning.
Form Validation: Checking on the user details and reverting back with a response.
1) Checking the user form entries: it saves time by validating the data types at the client end, rater than sending the data to the server and then validating it.
2) The use of banners and other visual effects requires user input and is displayed instantly with the help of a live script.
It helps in creating highly responsive web application rather than the old sluggish applications which required code submission for every function used.
Some very good examples which made this partial page updating web application (Ajax) expand its usage are web-based email applications. Google maps too helped in presenting one of the usages of Ajax.
Ajax uses by default uses XMLHttpRequest (XHR) object instead of the older objects like image requests, inline frames, and the script tag. This also plays as a disadvantage as not all devices and browser supports this functionality.