You can have Certify export the certificate to a location, either locally or remotely (ie SFTP) as a .pfx with a specified password, to multiple places if needed. (or most any format needed) Deployment tasks can be automatic or manual.
Outside of use for SmarterMail, you can have Certify bind certificates to specific service ports, such as DoT or DOH (in my application),
You can have Certify use a specific letsencrypt account with multiple certify installs which can be useful.
You have more control over the renewal frequency.
You can do DNS-01 Challenges to get wildcard certs.
You can specify that Certify use the same private key, or specify a private key, which can be useful for DANE applications.
Even if you let it generate a new private key every time, it's easily accessible if you need it.
You have much more control over the signing algorithm and options.
These are just the features I use, and I am by no means an advanced Certify user.
On the other hand, SmarterMail Certs are easier to set up and if they are set up right they just work.