They allow separate on page elements to be loaded on a page without the need for a refresh creating an easy to use page for customers. For instance, Kayak.com loads a list of prices and dates within the page based on your search dates:
Other examples include the infinite scroll we are all familiar with in Facebook or Twitter as well as ecommerce retailers such as Uniqlo which allow you to scroll through the entire Men’s inventory through the main category page:
AJAX Can Negatively Impact Site SEO
Different states of an AJAX application are typically denoted in the URL by appending a hashtag. For an example scroll down the current Flickr.com home page ; as you can see, though the page does not refresh and the URL stays relatively the same, a page section number is appended to the URL as you scroll down:
As a rule, search engines cannot read what comes after the hashtag, so they cannot know which version of the page to index.
Luckily, with a competent developer and the right amount of time there are a variety of potential solutions that can make your pages indexable to search engines.
Utilize Fallback Pages.
Create indexable “hashbang” pages.
Google introduced the hasbang (#!) in 2011 which translates the previously mentioned hashtag URLs into pre-set state indexable pages. However, this option is unpopular among SEOs due to increased development time and a lack of support from other engines and web services.
Create shareable URLs that replicate the current UX state.
It is possible to program server-side logic that will force a specific state on any page; for instance, think of shared YouTube URLs that load a specific moment in a video. These customized URLs can then be made shareable to allow external links to the page to tell search engines to index them. Again, the development time here does not necessarily justify the benefits.
Utilize pushState functionality.