Images are usually the main reason your web pages are not loading as fast as they should.
By default, when a web page is loaded, all the images on the page are loaded at the same time. This is irregardless of whether the page visitor actually views the images. As a result, the entire web page loads slower. The page visitor has to wait for the page to finish loading.
But what if the images are loaded only when the visitor views the images? As the visitor scrolls down your web page, the images that should appear will then get loaded.
This means that images are loaded on demand (as compared to loading all of them on page load), and more importantly, your web pages will then load much faster when visitors visit them.
This results in a snappier browsing experience on your web site.