Moderators can see deleted questions and answers in a user's profile, but there is currently no way for a moderator to see all deleted comments of a specific user. The ability to see the recent deleted comments of a user would be useful to quickly see certain negative behaviour patterns.
One example would be a user that is unnecessarily aggressive or rude in his comments, leading to those comments to be deleted by a mod or multiple flags. If the specific incident is not enough to cause a mod to annotate the account, or if it is entirely handled by the community, this behaviour leaves no trace in the user's profile.
What would be useful for such cases would be if deleted comments would be shown somewhere in the user profile, the activity tab would IMHO be a good place for it. Additionally, the ability to filter out flagged comments, ideally also by flag reason (e.g. only comments flagged as offensive/rude) would be very useful.
At the moment, comment flags pretty much vanish from view if you don't know exactly which post they were on. Being able to easily see the deleted/flagged comments from a user would help identifying certain behaviours and allow the mods to intervene.