If the whole team is working hard on the next major release and someone all of the sudden needs to produce a quick patch for the current one time may be of the essence. In this case some sort of a designator for "urgency" on a per review basis would come very handy for prioritizing which review needs to be done first.
In the most basic implementation this can be something like "normal", "when you have time", "I need this done ASAP". and can simply somehow reflect in the UI (like tinting in the list of currently opened reviews, or something like that).
In more advanced implementation this could be available to Herald, so, for instance reviews in a certain project get higher priority automatically.
In an even more involved support higher priority reviews can have their own policy for notifications (like they bug reviewers more often and go "stale" sooner).