I gotta tell ya, I only worked with them peripherally, so I don't have all of the particulars.
But a large percentage of the people worked unusual hours. The company required a lot of "ramping up" and "ramping down" of their employee base - for instance during their busy time the facility would be working three shifts, all out, for a month or two. Then within a matter of a week or so, the facility would be back down to basic 8-5 operations, or even just a skeleton crew over the holidays.
So, all of these "part-time" (I guess you could call them that) employees joined a "union" and paid dues into it. The union maintained benefits for all of the people on a year-round basis, regardless of whether the employees were on a full time, part time, or "no time" status. And that eliminated a headache for the company as well, and maintained a solid base of workers locally.
I probably don't have all of the specifics, but it made sense to me when one of the managers I was working with explained it.
Sounds almost like a union in name only. There are janitorial contract houses that do something similar, except they wouldn’t have exclusionary privileges in a geographic area like unions generally do. And the workers, however grunt and lowly, would be considered “management.”