It's not about that, it's about further security for the owner of the website.
The free SSL only comes with domain validation. The paid SSL certs come with a trustworthy certificate authority otherwise known as a CA. The paid SSL can be bought for 1-2 years, sometimes more, but the free SSL has to be renewed every 30-90 days manually by the site owner. Paid SSL's also require you to verify yourself as the site owner.
The paid SSL certs also offer support in the event anything happens. They also offer organization validated (OV) and extended validated from legit cert authorities. So, basically you get that seal of trust you see certain eCommerce stores offer. It gives the buyer a further sense of security.
Though I will say, the free SSL and paid SSL both offer the same level and type of encryption.