As an example the reporter field may require a value during the bulk issue operation. The reason why the reporter field needs a value is that this field is required. When you check the retain option, the reporters are retained where possible. Depending on your configuration this may not be the case for all issues being moved. For those issue a new (reporter) value needs to be set and that's why the reporter field needs a value set.

JIRA could be a bit smarter & user-friendlier and not require a value in the case when all issues will be able to retain the original reporter. It could also inform the user that if the value is entered and retain option is checked, it will only replace or set the reporter value for the issues that will require it. It would seem that at the moment the current message does not give enough confidence to the users, they fear that all values will be replaced.

Please see linked JRA-14385 for more details about the origin of this issue.