Social Proof is evidence that other people or organizations trust, use, recommend, or benefit from a business.
It matters because a website is often the first serious evaluation point for a buyer. Good design reduces confusion, builds trust quickly, supports accessibility, and helps visitors take the next step without feeling pushed.
Example
A project page can show an approved customer quotation alongside a real project image and a link to further details. The evidence should be relevant to that service and should not imply outcomes the customer did not report.