Hello David,
Thanks for writing in with your questions.
#1: I understand GD has a registration functionality for visitors. However, what is actually the purpose of this / benefit given that place ratings can be made also with user’s Name + E-Mail?
#1A The purpose is to require that a user submit a name and email to be associated with the rating.
#2: is it possible with GD that once a non-registered user has submitted a place rating he receives an e-mail where he needs to verify his rating by clicking a link? This would give maximum flexibility (no registration required) but avoid people just entering fake e-mail addresses and fake multiple ratings
That is not possible, as there would not be an email and an account. At least this is not a feature of GD, but may be possible with 3rd party plugins. Validation of email through an activation link is a optional feature included with USersWP. https://userswp.io
#3 Would it be possible to waive this rating e-amil verification if a user has logged in via facebook?
Social login registration does not bypass registration; it may be easier for users who are familiar with it, though, and could be seen as a positive feature for users who make use of it on other sites.