Because you don't want guests to be spreading their germs to everyone else's glasses when they forget which drink is theirs at your birthday party.

Your at a one of those party's where there those plastic cups that all look the same.
You put down your cup at a table with a bundle of cups that look the same as yours.
So eventually you guess which one is yours, and pray you don't get hepatitis.
Well,
Now you wont have to guess which cup is yours!

Bring this with you to a party, or when you throw a party! (via dailycandy.com)

Nice way to keep things straight at a party and help the environment at the same time...