I've known a few OWs like this. In some cases, they were full-fledged mistresses. With the latter, there is an imbalance because the man mostly sets the pace - they get pissed off if the 'mistress' cancels or is unavailable too often. But the payoff's good. They get a nice flat, nice presents, a relationship without the daily grind or family/budget obligations and, when it comes to an end, they get to keep the flat and the gifts.
In those cases, I imagine the wife must know. There's an odd kind of safety in it, I think. Nobody's trying to break up a marriage, and there's obviously enough money floating around to fund the main household and one or two small ones. If you're married to someone who won't be faithful, this is one way to keep things stable.
Other long-term OWs I've known mostly did it because they're very independent and genuinely don't want to enmesh themselves with a partner. It's really not the case that every OW hopes to get 'her' man away from his wife & family: in these cases, the OW prefers the man to stay married because she really doesn't want any bloke badgering her for more commitment.
The second-weirdest setup I've encountered was a couple I worked with. Both married to other people with children, family holidays, school events, the lot, and they'd been having an affair for over 20 years. Still are, from what I gather. They're deeply involved with one another and also with their home relationships. Looks like a lot of hassle to me, but it made them happy!
The weirdest, to me, looks like double hassle: men with two families. One of my relatives did this and, apparently, it's way more common than you'd think. With my relative, the OW had hoped she'd be The One True Love who would get him away from his wife. Eventually, however, she settled into being a semi-single parent. His wife never truly forgave him but they stayed together in what most would consider a happy, successful marriage until they died.
People do odd things 😂