It's pretty common (but not the majority) for young(er) people who are either early in their careers or who are established in low-wage but valuable careers (i.e. teachers, social workers, EMT, vet tech, etc.) and work second jobs for necessary income (as opposed to disposable income). It seems more common in metro areas because the cost of housing is higher. It's also more common with single people than it is with people who live with a partner or a roommate. I've also seen people do things like this to cover additional expenses that can't be filed under frivolity or "bad with money" (living alone instead of living with roommates, becoming a single earner household while their partner goes to grad school, paying child support).

There are lots of these people on Reddit since Reddit skews younger, so you see them overrepresented here. They are not the majority, but they are common.