This is a good idea that we are also looking forward to use.
Here is a current use case we are facing : we are working on a show that contains several episodes, and we are working on different episodes simultaneously. When creating a client review that contains shots from different episodes, we would like to be able to group those shoots depending on the episode they belong to.
Right now, we can instead create a client review for each episode, but as AlbertoGz said, this would lead to a huge list of client review thus a huge number of associated mails.