Yes - just as there is less (assuming an HONEST relationship, which is even more critical in poly relations than in mono ones) risk of disease when you are sleeping with just your husband - having multiple spouses affords that same 'protection' with multiple people.
It's called polygyny, and there aren't many. I think there is a tropical/island people that practice it, but don't get me to lying.
My good friend is the girlfriend, but I know the wife/husband pair as well. They have always been in an open marriage, and A is the first woman that they have been able to integrate into their marriage smoothly.
It's interesting that you ask the reason for doing it - I asked them (GF & wife) once why, and the wife's answer was more along the lines of 'Why not?' - she enjoys having another adult member of the family, and they aren't in a monogamous marriage - never planned on being in one - and their relationship works, for them. That's about as far into their inner workings as I've gone - I don't know how they divvy up the coloring, and I'm not sure I want to.
I think W is bisexual, I know A is - but I don't know if they are coloring with each other, either - though, it would surprise me if they were
not - they've got a certain energy.
My observations? Wife & GF were friends before the relationship with H came into the picture - so that helped things, a lot - she wasn't a 'stranger' so to speak. I don't know who initiated - I suspect H did, but it wouldn't surprise me in the least if W approached A.
Their little girl (H&W's) loves A, and considers her a second mother. A isn't interested in children at this time (she's still in school), but she enjoys taking care of R (the daughter) when she can - and H&W love having someone they can totally trust to be with their daughter in their house.
The community (pagan) doesn't care, as long as everyone is being ethical - ie, being poly is okay, cheating is not. I don't think that they live together, but they spend a LOT of time together - W&A on their own, H&W on their own, H&A and HW&A......