>>716204859
If Deltarune only has one ending, and weird route will just give you a worse ending, then my guess is that the most you'd get out of Shadow Crystals are additional content with extra lore and interactions. You don't even need the Shadow Mantle to beat the Knight, but doing so will make the fight easier, and you get to learn what happened between Tenna and Spamton.
The same could be said about the eggs, but we're just learning about Kris.
