In a word, access. When one struggles to come up with items for a tough differential diagnosis, it's not usually because one is dumb or because one is ignorant. Generally, it's because the items for the list are scattered t hroughout one's brain in dusty little cupboards. A mnemonic is a technique that let's one sort through these cupboards in an orderly fashion. This process can generate a respectable differential diagnosis, even for findings that one doesn't often encoun ter.
Mnemonics operate most efficiently when they are designed for a very specific finding.
However, not all mnemonics are created equal. Some are long, some spell words that don't exist in any known language, some are pretty darned contrived, and some are pretty darned trivial. Somewhere toward the end of my radiology residency, while studying for radiology boards, my mind began to rebel at the thought of having to learn even one more mnemonic. One solution for me was to just learn just one more really good universal mnemonic, and to use that for everything else. Below, I've applied this universal DDx to pulmonary disease and to musculoskeletal disease.