What's with the coins?

[Beelzeebub]

I see a lot of people using a coin as a reference in pictures when trying to show tire tread left. But i really don't get it. How does a coin show tread remaining?

[wannabeS4]

here you go

http://www.tirerack.com/tires/tirete....jsp?techid=51

[Beelzeebub]

thanks