The decision as to who should, and who should not, have spots was made by a higher authority long before I started to take an interest in deer stalking.
He quite forgot to consult me on the matter and has never explained the reasons so we can only make an educated guess based on logic and common sense.
And when has that ever had anything to do with the mysterious workings of Nature?
However, I’ll have a go.
First, deer are not alone in having young covered in DPM (Disruptive Pattern Material) or what we usually call cammo.
The young of wild boar, for example, are like little stripy humbugs, even though with such parents they probably don’t need a lot of camouflage, and the chicks of ground-nesting birds, like their mothers, are usually mottled, striped or spotted.
So I think that must be the purpose of the spots and stripes; to confuse and blur the outline of the little creature, making it less easy for a predator to spot it. This seems to be restricted to animals and birds that live and breed on the ground, so the DPM/cammo theory is probably correct.
As they grow, they rely for safety on alertness and speed so the spots, being no longer needed, fade or disappear in most species.