Benefits that I see for my family are that there are two strong men to protect and provide for ourselves and our children, thus taking some stress off each of us individually. It also makes it even more practical for me to work from home. While I make only half of what each of my men makes each year, I am home when a child is sick, I happily do the laundry, dishes, cleaning, even mowing the yard, so that when my men come home, they are free to relax and do things they love to do instead of having to share in household chores. They can come home to a beautiful home with happy children and dinner on the table.
As the poly person in our V, I also feel the completeness of loving to my capacity (or at least close to it) that Redpepper spoke of. I have been in so many relationships where my partner felt like he was running himself ragged and would eventually come to the conclusion that I was, “too much woman for one man to handle!” At which point I would tearfully leave said partner in search of that special person who COULD handle me all by themselves; only, that person really may not exist.
I also have a sex drive that most men over 25 have trouble keeping up with, so there is another practical way for them to “share the burden” :-)