The certificates are available at no extra cost.
In their announcement Pressable explains why this is an important feature:
Back in August, 2014, Google announced that sites that have HTTPS enabled would achieve better search engine rankings than sites that don’t have HTTPS enabled.
With the security aspect being as important, if not more:
Today, HTTPS encryption is practically a necessity for WordPress sites. This is due not only to search engine optimization (SEO) considerations but also to the fact that sites that are encrypted with HTTPS are more secure than unencrypted sites.
The team at Pressable have made configuration easy – customers just need to log into their control pannel and flick a switch to enable it.