Had a fun panel once at Philcon where I answered the questions posed in the description in the first minutes, and we still had a ball. It asked why a magical sword is only logical but a magical toothbrush is fun.
There are two reasons, really, and the minor one is familiarity. Magical swords are age-old. Magical toothbrushes would be new. A magical car is still hard to conceive of, but a magical train is fairly common -- as long as you hit the major reason rightly.
The major reason is there must be some suitability between the purpose and the object. Some works of fantasy are weak on this, and it weakens the story. A sword that cuts anything, or just about; that inflicts wounds that do not heal; that requires blood to be resheathed, magic not needing to be nice -- all of these fit swordly nature. A sword that shoots fireballs is pushing it. A sword that translates foreign languages is really pushing it. A train therefore should be a means of transport, however magical; send it off to Fairyland -- or the afterlife -- rather than let it cure the land of its curse.
We spent the panel riffing on this magic item and that one, discussing the metaphorical suitability of each to its type. Rings are popular because you can put on things, such as invisibility, with the ring, though some writers really push the sorts of abilities involved. Mirrors, showing faces and forms, are the logical sources of doppelgangers that move on their own and perhaps are other than what they reflected. Keys unlock things. Or, of course, lock them, or both. A horn would send a message of some kind.
And we interchanged it with trying to devise a magical toothbrush. We finally concluded that it was made from the bristles of a boar out of Celtic legend, and its effect was to make everything you said sweet and eloquent.