Acquiring an SSL Certificate and applying it to your domain will make your domain change from HTTP to HTTPS. The main reason for doing this is for increasing security on your site and allowing for secure payments.
Many people used to think (many still do) that transferring your site from HTTP to HTTPS would lose you a lot of that important linking juice but that isn’t exactly the truth.
If you take a look at the top ranking sites for any search query on the web, I guarantee that 99.99% of them (if not 100%) are HTTPS and SSL Certified.
This shows that Google ranks those more secure sites much higher than those without. This is because the sites are more secure and the promotion of non secure sites would not work well in Google’s favour.