Why does this custard say to boil milk cool and boil again I have this recipe (from a really great book that has never failed me) for a custard and it says to

Bring the milk to the boil.
Put to the side for 10 mins.
Whisk the yolks and sugar.
Return the milk to the boil.
Add milk to eggs.
Heat till thick.

But why the double boil step?
It has made a lovely thick custard, my best recipe yet.
Edit: I have eaten it now and it was the most scrummy custard I have ever eaten. but that may be the 4 egg yolks and double cream :)
I can't say about this particular recipe, but 'scalding' milk was a commonly used to change the milk (cooking proteins, deactivating enzymes, etc) in the days before pasteurization ... but that was normally done when the milk was to be used at a non-boiling temperature.
It's possible that this 10 minute cool down gives it sufficient time for the desired changes to the milk to happen, without the problems of boil-over and evaporation that might happen if you tried to hold the milk at a boil for an extended period.
It's also possible that this is related to another question on here in which someone noticed that re-boiling milk was less likely to foam up.  (but then couldn't re-create it, so there might be something else going on)