Rich footers include more information or functionality than simple footers. Besides social media and utility links, footers can contain other components that will depend on the website's type and purpose.
This could include:
Global navigation on long pages so that users don't have to scroll back to the main navigation
Secondary tasks like applying for a job, finding company affiliates, etc., on sites with multiple user groups with different user journeys
A site map on large sites with multiple levels of information or subdomains
Testimonials or awards for startups or companies with less brand awareness
Brands associated with large, multinational organizations with many subsidiaries or partnered brands