You use filename when you want to use the full file name, you use frame when all you are interested in is the frame number
Note the frame number might be present as metadata in the file even if the filename itself no longer has it. I rename all my files when ingesting, PM at that time stores the frame number in a (PM only) location in the metadata, allowing me to look it up later.
Note thought that as the frame number only goes to 9999, the number is quite useless to me; I easily shoot way more images than that, plus if you shoot with multiple bodies, you'll get mixed numbers anyway.