Tempering the egg mixture is extremely important if you want to end up with a smooth custard! Tempering eggs means to gradually heat up the eggs instead of shocking them all at once with a hot liquid. If you shock them, the eggs with cook and curdle and you will have scrambled eggs in your custard (not what we want)! On the other hand, if you add the warm liquid slowly while whisking, you are incorporating air into the mixture and allowing the temperature change gradually. Resulting in a smooth egg mixture.