Framing hammer gives more "grip" when striking a nail.

Finishing hammer won't mess up a delicate surface when struck.

They both drive a nail but used at different times. Could you just use one hammer? Sure, but it is harder to drive in large nails with a finishing hammer and a framing hammer will mess up your trim. You'll mess up your finishing hammer if you strike framing nails with a finishing hammer.

A flat iron or a hairstyling iron is a specific tool to straighten hair

A "hair straightener" could be a flat hairstyling iron but it could also be something that doesn't "iron" to straighten. It encompasses more.