To store custom data private to the page or application.
The "data-*" attributes are a set of custom attributes provided by HTML5, allowing one to store custom data specific to individual elements. This embedded data can then be leveraged by scripts to create a more interactive user experience without any AJAX calls or server-side database queries. It's a way to store extra information directly in the markup, without resorting to non-standard attributes, invisible content, or other hacks.