Could be wrong but I'm pretty sure at least one or two allowed you to come back with seniority (maybe with a loss of the weeks/months you were gone, which is common with some other types of LOA).

Makes sense, especially with eras where people got stuck for years or decades by industry timing. It allows older senior pilots (who have families, mortgages, lives, kid's college looming) to take a shot at moving up, without betting their family's future on a training event. It's not a trivial decision, BTDT, and I had relatively recent training events and also the military kept my brain more engaged than say someone at a single-type regional who upgraded 15 years ago.

Makes sense for the airlines too, regionals clear their books of expensive lifers, and majors get more access to experienced pilots.