Sessions are the killer feature for me. I have a session for each project I work on and can have all my tabs, panes, vim, etc set up and switch between them at any time. Sessions can be persisted so that even if I reboot my machine all my project workspace configurations are saved and ready to go.
The session doesn't actually persist between boots. I think what the parent must be referring to is the ability to create tmux session configurations. Then after a reboot if you try to connect to the session which no longer exists the conf will spin it up for you.
