By adding an email address you resume playing across multiple devices and web browsers.
Otherwise, all your accomplishments in the game will only be accessible in the browser session where you started. Further, if that browser crashes or its session expires, you will lose access to your game data and will not be able to resume where you left off.
If you have gotten some entertainment from playing, we ask only for a small favor in return. Though we have enjoyed creating this game, it did take a lot time to develop and costs actual money to keep the game running. Joining the Plates Club does not cost much, and we hope the added features make the game even more enjoyable.
The simple answer is: privacy. Companies that provide "free" games have to make money somehow and they do this by selling your personal information: your email, the games you like, the times of day you play, how long you play, and even scarier things. The "free" game companies sell this to other companies that are compiling a database of all your actions from other games and web sites. Your data is then sold to companies and agencies that use it to manipulate and exploit you in ways you might not even notice.
For those "free" games, they are only free if you do not put any value on your privacy!