Because flies growing at different temperatures emerge at different sizes, this explained an awful lot of the variation in shape and size which previously had been thought to be genetic. And, in turn, that helps explain another large but largely ignored question in evolution. For most creatures, it pays to be big. It makes you a better mate, better at dealing with enemies, better at coping with heat and cold. I also have a vicarious interest in sex.
Nobody really knows why sexual reproduction is there.
About the only way to study it is to look at those few creatures who've given it up. Some, though, have taken the easy way out. They fertilize themselves, effectively abandoning sex altogether. Their genes show that these species are essentially a mass of identical twins, with no diversity at all. The pivotal influences on my work have been Bryan Clarke and Dick Lewontin.