Safari Security wants to use confidential
All your credentials are encrypted and stored using Keychain (e.g. ~/Library/Keychains/). It provides secure storage of your passwords, keys, certificates and other private data.

To prevent unauthorized access to your credentials by 3rd party application, each time when keychain file is accessed, it asks for user permission to access his sensitive data.

If it wouldn't ask you each time, then any application (e.g. Safari plugin) or hacker could access your keychain file and steal all your passwords without your knowledge by a single command.

Read more: Examining Mac OS X User &amp; System Keychains