Moderators can annotate users (on the main sites), those annotations are only visible to other moderators and the SE team. This is very useful for a moderator that does not know the history of the user. Previous behaviour plays an important role in moderator decisions, and it can decide whether the moderator just warns the user to stop that behaviour, or suspends a user that has been warned previously and should know better.
On chat, nothing like this exists. Moderators can see a list of previous flags (which is rather hard to interpret and doesn't even tell us if the flag was validated or not), but even previous suspensions are not visible to moderators.
As chat flags are global, it is very likely that a moderator unfamiliar with a specific user will act on flags. In such cases, having annotations available would be very useful to quickly get up to speed on any history the user has. As it is now, many moderator decisions are based only on the flagged post, and maybe a few posts before that, but ignore the history of the user as the acting moderator often just doesn't know it.