If what you mean by staying with one partner means to be married to one spouse, the benefits are immense. you get peace of mind, you enjoy yourselves better, you get to know what love truly means, you trust each other, you focus on making one another happy, etc.
But if what you mean is having just one sexual partner though not married. There are no true benefits. It is just that it is not as disadvantageous as having multiple sex partners, but it's still the same thing. If u have a sex partner you're not married to it simply means you're not committed and u can break up anytime to get another sex partner. To me it's still the same multiple sex partners at the long run.

hope u all practice what u preach