Some users put their timezone with location. For example
India (GMT+5:30). It helps other to understand availability of an user.
Can it be an optional field in user profile?
I don't think general users are interested in knowing when a specific user is available here and that shouldn't be an intention too; for all generic purposes. Still, specifying your location is optional which can imply your timezone, if they are interested anyhow.
So adding the field is hardly worth the change.