So Kylo retrieves a fist-sized chunk of Hosnian Prime and (tentatively, ceremoniously) gives it Hux, and Hux is just like “Kill trophies, Ren? Just because you bathe in the ashes of your deeds doesn’t mean the rest of us want to, you disgusting creature.”
But there’s something oddly… compelling about having a tangible memento of the indelible mark he has left on the galaxy–a glass-metal chunk of what used to be the living, molten heart of a planet–something he can touch, cold and heavy in his hand, and think I did this. It cements him in time, in a way–he only has to look at it and he is on Starkiller again, young and proud and unafraid, watching his legacy scar the galaxy with a blade every bit as blood-red and blinding as a lightsaber.
(He only has to look at Kylo and he is on Starkiller again, small and broken and terrified, searching for a piece of wreckage far more valuable to him than any chunk of rock could ever be. He thanks the stars he found him.)