Once you're happy that what you have is good melted chocolate, the final step is tempering.
Chocolate is a suspension of finely ground flavorful things in cocoa butter. And cocoa butter is, chemically, a complex mixture of different fats. It can take on various crystal structures, depending on things like the exact cooling profile, agitation, the presence of seed crystals, etc. And these structures can have very different properties in terms of mouthfeel, stability, melting temperature, etc.
If you just let melted chocolate set back up again, it won't be nice. The default crystal structure that the cocoa butter adopts will get soft and sticky at even moderately warm room temperature, and has a gummy feeling when chewed. It will also develop grey, powdery bloom very quickly, and turn chalky over time.
So instead, you cool it *past* its normal freezing point, but with continuous agitation. At another, lower freezing point, it starts to form better crystals.
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