Not everything is a phpBB forum.

Also:

> Select Google Account email. If you can't open this setting, it might not be possible to change your email or username.

> If your account's email address ends in @gmail.com, you usually can't change it.

https://support.google.com/accounts/answer/19870

Say, for example, systems that directly expose your email address as the identifier that can be used to reach you. "Send money to saagar@saagarjha.com [using our service]" kind of sites.