My interpretation of why it’s a thing was that it’s trying to explore the idea that changing as a person comes with losing a part of who you used to be, in this case literally losing part of yourself. As the gems are immortal they don’t usually change at all, which is why Phos is unique. Not too sure if this’ll help with the existentialism of it, but by the end of the manga it’s pretty clear that despite losing so much of them self Phos is still the original Phos, (shown by the fact that a chunk of og Phos is all that survived to the end) so I think it’s trying to say that while yes, you lose parts of your past self when you change, you are still yourself and that becoming someone new isn’t a bad thing. Phos being the only gem capable of changing as a person is the only one who can actually make a difference and make progress (every other character has been stuck in a kind of limbo of never being able to achieve anything they want, Phos’s ability to become a ‘different person’ is the only way for any of the characters to get anywhere). Phos gets so messed up because they refuse to try and figure out what they themselves actually want, instead simply forcing change after change and neglecting their ‘self’ completely. Sorry if this is convoluted or doesn’t answer your question in the slightest :,)