Beauty and the Beast has a lot to say about our inner worlds and the way our true selves sometimes disappear beneath the surface impression. Gaston, for example, is beloved because he's superficially handsome even though he's pretty nasty underneath. In the castle, the servants' real selves, the humans they once were, are hidden beneath the shape of household items, while the Beast's former handsomeness vanishes beneath the whole lycanthropic land-cow thing.
But, unlike Gaston and Belle, the servants' transformations clue us in to what those real selves were like pre-curse:
- Look at Cogsworth, for instance: fussy, persnickety, always about to pop off in a snit. Small wonder, then, that he'd be turned into a clock.
- Lumiere is glowing and friendly…hence, candelabra.
- Mrs. Potts is comforting, reassuring, and warm, just like a good cup (er, pot?) of tea should be.
- The Beast, of course, becomes ugly on the outside the way he's always been ugly inside, and he can't earn back those male model looks until he's got the soul to match.
In every case, the changes are intended to convey a lesson: learn to recognize the souls of those you meet and look past the superficial. By being forced to wear their souls so openly, the castle residents have their outsider status reinforced by the curse, which also suggests how corrupt the supposedly "normal" world is. The residents of the town can hide themselves, after all, while the castle's residents can't, making the townsfolk more capable of deceitfulness like Gaston.
Belle serves as the antidote for all of that.
She's the only character who's beautiful both inside and out and who doesn't show any difference between her inner self and her outer self. That's the harmony she brings to the castle, and the gift she gives is providing the peace and acceptance to make the Beast as beautiful as she is (and bringing the rest of the castle along by default).
The transformations last only as long as they need to—long enough for the Beast to learn what Belle is teaching him and for the rest of the staff to follow suit, returning everyone to their normal forms but leaving them just a little wiser as a result.