Chocolate scorches quite easily and will impart an aftertaste and leave your chocolate less than at it's creamy goodness. This also causes the cocoa butter fats to separate and causes your chocolate to 'bloom' - that whitish looking stuff that you may see on a bar of chocolate that has melted and then hardened again. Removing melting chocolate from the heat before it's fully melted allows it to have enough time to melt evenly while cooling down before it scorches. This is tempering chocolate, which gives it a glossy finish and nice snap when it's bitten.