A wealthy man can put his girlfriend in a flat, pay her rent and expenses. He can see her when he can, and usually phones ahead to make sure everything is ready; i.e. a meal, a bedroom, etc.
In this way a woman has a part time annoyance who is economically valuable.
In many of these cases the wife has reached near loathing point but money and position prevents divorce.
Often, these women know all about the girlfriend but pretend they don’t.
With an older man, the hope is that he dies soon and the wife can get everything without him.
In other cases, the wife doesn’t have a clue and the husband works hard and keeping it secret.
The Other Women like this because it cuts down on the time she has to be with him as most of these women have their ‘side man’.