To understand why you might want to use tmux try the following:

1. Open your terminal
2. Start editing a file with vim or nano but don't save the file
3. Close then re-open your terminal

You will have lost your progress, next we can repeat but this time using tmux so you don't lose your session:

1. Open your terminal
2. Start a tmux session using tmux
3. Start editing a file again using vim or nano
4. Close and re-open the terminal
5. Type tmux a to re-attach to the existing session

Note that this time none of your progress is lost.

Aside from enabling you to have a persistent session, tmux also allows you to have multiple terminal panes open so you can do more than one thing at a time in the window, to see what I mean try this:

1. Open your terminal
2. Start a new tmux session using tmux
3. Type top to begin listing processes
4. Press ctrl b then % to make a new split pane
5. Enter ls or other terminal commands

You will see that you can use more than one panel to do things. This can be useful for example if you want to watch run tests and also run other commands.