Moodle allows you to delay the release of students' grades so that they can become visible to students at an appointed time. For example, this might be useful to you if you're grading assignments over a series of days and want all students to see their grades after all grading has been completed.
Keep in mind that when you hide a Moodle Assignment grade in the grade book, it is hidden throughout your course page (even when the student looks at their assignment submission). Although the grade will be hidden, the assignment activity link will still be visible.
Another option in Moodle to release grades is to set grades for a single assignment to automatically become visible to students at a scheduled time.