Proper leather shoes

Why 'must' they be cedar? I get the aroma repels moths but do their eggs attack leather, or is it just another line of defence?

Moths just "follow their noses" when it comes to cedar, lavander and a number of "natural" repellents. The oils are not poisonous to moths, their eggs or larvae. Cedar and lavander oils have a strong pungent smell and the reasoning behind their use is that the smell will overpower the scent of wool. There is, unfortunately, little scientific evidence to support this theory.
Cedar's main function when it comes to shoes is to "overpower" the smell of poorly made shoework.
Are cedar shoe trees of any benefit? Their main benefit is cost. Cedar wooden shoe trees are cheap to make.
Are their any downsides to cedar? Yes. Most cedar trees are poorly made and quite rough. All woods absorb and displace water. Most cedar trees are not terribly stable. Cedar seems to be an American marketing thing.
The best shoe trees are made as lasts for the shoe. I only have one of these and they are nice but it came with a pair of handmade shoes that I bought over 20 years ago (and they were literally the last of a breed).