Adding an expiry date means that we'd eventually need to input our login details again. 

Can you think of a reason for adding an expiry date?

Safety To increase visits to the website

Awesome! Without adding an expiry date, the authentication cookie would be valid forever. Anyone using the device could access the account. 

The expiry date is added as a layer of safety. It ensures the person logging in is the person who is authenticated in the cookie.