If you are connecting a lot from via SSH to remote servers, entering passphrases over and over might be a pain. As long as your hardware is secured, you might consider enabling the macOS keychain also for your SSH connections.

Go to ~/.ssh and open or create the config file. Now just enter the following entry to the file.

Host *
    UseKeychain yes

In the case you want to enable the keychain only for a certain server, simply replace * with your hostname such as foo.com.

Prior to macOS Sierra, macOS offered a dialog to enter the passphrase where you were able to select the passphrase to be remembered. For some reason, this dialog was removed. On the other hand, the keychain option has been introduced with macOS 10.12.2.