Signing in to your Google Account lets you synchronize Gmail™/Google Mail™ and your Google contacts and calendars
between your phone and the web.
You also need to sign in to a Google Account
to use Google applications such as Google Talk™ and Android Market™.
If you skipped signing in to a Google Account
when you first set up your phone, you may encounter problems when using some Google
applications. If these problems persist, you may need to factory reset your phone and
enter your Google Account details when you set up the phone again. To factory reset your
phone, see Performing a factory reset (hard reset)