The main reasons are three:
The first is security, as all data on a website with a security certificate installed is transmitted via a secure HTTPS connection.
The second reason is sales, as web users are increasingly paying attention to the security lock icon and HTTPS connections, which is most visibly reflected in the increased sales of secure online stores.
The third reason is closely related to the visibility of the website or its visibility in the Google search engine. For Google, an SSL certificate is a positive signal that contributes to higher website rankings for individual search queries.