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Although there aren't necessarily any current features that would make use of such a field added to the xf_admin_template table, there are circumstances where maintaining the last_edit_time of admin templates would be useful.
The date doesn't actually provide much information that we can use programmatically (at least with the public templates). A version number provides more useful data.
You probably know what I'm using it for. For public templates I am making use of that value externally.
I would like to do the same with admin templates which would nicely complete the add on.
Aside from that a useful core application for this could be displaying a list of recently edited templates as an alternative to listing them alphabetically.