Required has a UI impact putting a red asterisk next to the field. If a user cannot edit the field the red asterisk would be misleading as they'd try to enter data in a field that is locked.
Presumably the calculated field is populated by a business rule, in which case this will run when the BO is being saved which will populate the attribute unless you create with the NO RULES NO VALIDATIONS flags set which will override the required flag anyway.
If the attribute is not populated by a business rule at the point of creation but rather some input from a process then you could enforce its required status by adding a business rule - but in this instance I don't see how An end user would be able to create a BO.
