main use of process backed record

That is rarely used as we persist all relevant process data to DB. Did not do that in recent years since SAIL and Records was introduced in 2015.

Yes, as best practice is to archive/delete your process instances on a short schedule, and process backed records only work on processes still in memory (not archived), there are not many use cases for this anymore.  I don't have any.

The only time I use process reports is in a few situations where I need to check a process variable contents of a running process, which does not occur often since we can persist to the DB and check there.