You can reuse OpenSSH connections to the same server when you want to open subsequent connections to the same server. For example, if I ran ssh email@example.com, the ssh client will establish a new SSH connection. It usually takes only a few seconds. However, if I run ssh firstname.lastname@example.org command again, connecting to a server42.cyberciti.biz multiple times result into the overhead.
This tutorial talks about ssh features called multiplexing that reuse an already-established connection when creating a new SSH session.