Ok. Most of the major integration components work (profile attribute and audience sharing) work with at.js, but the attributes and segments are not available until the next session after authentication with at.js. It's much faster with Web SDK, but not required.
Here is one way to think about the business value. If you are a Target power user, you are probably familiar with the usefulness of Target's profiling system. You can collect and store info about the user across sessions and then use it for personalization. For example, you could have a Target profile capture the last product they viewed in a session and then show them a personalized message with that product on the very first page view when they return to the site. With the Real-Time CDP integration, assuming it's a known user (people authenticate on your website), the profile becomes much larger. Not only can you profile based on browsing behavior, but you also have access to attributes from additional, non-web, non-mobile sources like your CRM, call center, loyalty system etc.