They suggest assembling the project with a socket so that you can take the microcontroller off and reprogram it in an external programmer.
Another way to build it with a socket is how we’re demonstrating here, in this test fixture for the project.
This setup has a tiny piece of perfboard that connects a 20-pin DIP ZIF socket and a socket for the 18-pin LED display. Thus, both the AVR microcontroller and the LED display are socketed, and can be easily removed. This fixture is used primarily to test programmed AVRs. Since the chips are swapped out regularly, the ZIF socket makes sense, even though it’s more expensive than a regular DIP socket.