I was an adjunct for a semester at a Big Ten university, many years ago. Like you say, there's usually a reason, such as collecting benefits while running some kind of side hustle. A teaching gig lends itself to this because the hours are flexible (outside of your scheduled class time), there is utterly no supervision, and no questions asked about what your other income sources are.

My office mate in engineering was trying to get funding for a start-up. I was trying to get a consulting business off the ground. Neither of us achieved those things, but whatever. He got a teaching gig at the community college, which is unionized and actually a pretty good situation. I found a regular day job through his network.

A friend of mine had an adjunct gig in the humanities, and used his off-time to learn how to code.

A lot of academic spouses get adjunct gigs, especially if they want to balance part time work with child care.