Sometimes the motivation in filing a proof of claim in the mortgage records is to put third parties on notice that a claim exists against the deceased’s estate. That notice could be general in nature, not necessarily just applicable to real estate. The notice can include a request that the Clerk of Court provide notification in the event a succession is opened.
Any “interested party” can open a succession -and that includes creditors. Sometimes a creditor may doubt that the deceased has enough assets to justify the creditor going to the expense of opening the succession, so they will go the claim route.