Corruption. It's all about sanctioning fees.
Whatever other reason they gave for forming those weight classes is bullsh*t.
That's why most of them have come along during the proliferation of all the title orgs.
Several of these divisions aren't very old.
Jr. Lightweight and Jr. Welterweight I believe existed in the 1940's or thereabouts, and then ceased to exist for another 20 or 30 years until being brought back.
Some guys would still fight weighing 155 or whatever back then. It just was a Middleweight bout.