To be able to exist List permission level details si something that can be very handy.
The organizational scenario is migrating sites from on-premises to SPO. The old site needs to be made Read Only after cut over. During UAT of the new site in SPO, some items will be over looked. Then after cut over, a major issue may be discovered. One may need to revert to the old site until a resolution is implemented.

Another scenario very similar is for a Classic SPO to Modern SPO. in this scenario, it may be a sub-site being promoted to a Modern Team (therefore a site collection). Again, the business may be unhappy for some reason and we need to revert.

Last week, a colleague accidentaly deleted an AD-group form a site collection. We had restore a backup in another environment, make a Permissions Matrix Report an add the group manualy on each site/list that didn't inherited the permissions. An import of that report would have been very handy :-)
It would be very handy too, to have a powershell-command to generate the Permission Matrix Report in order to automate the export on daily, weekly or monthly base.

Possible idea : allow the exporting of permissions from the existing site/domain and then (after we have migrated all the data and moved the new domain) re apply the security on the migrated content and structure.