To make sure that the pictures of your domain are displayed correctly on social networks like Facebook and Twitter, these social networks have published guidelines on how to set a thumbnail. This makes it possible for every user to get a good and correct preview or impression of your page in social networks.
Furthermore, if you share your website in social networks, you want other users to click on this link. A preview image is clicked more often than just a title with a description. Don't just use any picture on your website as a thumbnail. Think carefully and consider which picture could be clicked most often by interested parties.