As a website publisher, you want to show your visitors personalized content or content recommendations which fit his historical interest. By passing through a unique identifier (via the parameter customData) such as an email address, a hash or fingerprint, an advertising ID or any other UUID (Unique User IDentifier) you can easily enrich the user profile with contextual data and get information about the categories, keywords and other information the user is interested in. 

Knowing what a visitor of your website is interested in helps you to provide a unique user experience to show relevant and similar content to each single user.