Not all projects require Mandatory Review, but carrying out reviews of contributions can increase confidence of contributors. Contributors do not want to unintentionally introduce errors, so they may not submit a contribution if they do not have confidence in it.
Allowing a contributor to request that another contributor reviews their change provides an opportunity to introduce Feedback Loops into the workflow for maintaining your data.
A Changeset that has been flagged for reviewing might be immediately applied to the database, so the review is done retrospectively. Or it might go into a queue for reviewing before being applied.