In desserts like panna cotta or crème brûlée, why do you always boil the milk/cream? What does boiling do to the milk?
I am not sure that the boiling step is absolutely necessary, but it is definitely an easy way to heat to a known temperature (rather than saying "heat to 180 degrees" and people complain because they don't have a thermometer). Or, it could just be because generations of chefs have done it that way and nobody thought to ask why. Either way, it is typical to heat the dairy, temper the eggs (creme brulee) or dissolve the gelatin (panna cotta) with that, and then finish cooking (gently heat to the desired consistency) for the creme brulee or just chill the panna cotta.
With that said, I have successfully made thin custards for ice cream/ gelato sous vide without first boiling the dairy and they have received wonderful reviews from my wife. Also, this recipe for panna cotta specifically cautions you to not boil the mixture.
Creme Brulee, panna cotta both involve mixing sugar (or sugar and honey) with the warmed milk. This is generally true of other similar desserts as well. Heat is a catalyst to the mixing process. The heat aids in dissolving and mixing the sugars into the solution. If you try to mix either in cool (or cold) milk most the sugar will clump and rest at the bottom and the honey will separate from the milk.
I think that this process (scalding) was done before there was homogenization and pasteurization of milk as a general rule, and that heating the milk would kill off any bacteria, and do some de-naturing of the proteins in the milk, making a smoother sauce or custard.
Since most milk is pasteurized and homogenized these days, it is an unnecessary step in most custard or sauce making.
Scalding milk destroys some enzymes so that it improves setting, as in custards.