Writing the filename into the IPTC title field is a really useful practice. It preserves the name in a place that’s accessible, and likely to survive any kind of renaming I typically use it for one of three reasons.
I like to write the name in the title field of all my images after they get their permanent name in the ingestion process. That way, if a client renames the file and asks for the original, I have a breadcrumb trail back tp the original. All I need to do is look in the metadata of the renamed file.
Another reason to do this is that you are using a service of some kind that needs the files renamed. Pictage is a popular wedding print service that requires files to be renamed or upload.