Would you tell your GMail password to a friend? Your colleagues in the office? Publish it on the Internet?
The reason is that Google uses something called a “token” to allow apps your smartphone to connect to Google services like your mail box, your calendar, etc. The token is like a key on your keychain: Anyone who has the key can open the door it fits. Unlike keys on your key chain, anyone who can pick a token out of the air knows where that door is!
Related article: Catching AuthTokens in the Wild