The English string under the real name field in [[Special:Preferences]] (prefs-help-realname) reads:
Real name is optional. If you choose to provide it, this will be used for giving you attribution for your work.
Afaik, the only attribution that uses the real name field is the "Edited on 1 January 1970 00:00 by Joe Bloggs" message in the footer.
This message isn't shown if $wgMaxCredits is set to 0, so the help message is misleading: we should squelch this message if $wgMaxCredits is set to zero, or modify it to list other places we use the real name (for example, salutations in emails).