For us, the biggest is financial protection.
We're in a 1950s household and my husband/Dominant busts his ass to ensure the financial health and well being of his wife/submissive and home.
By doing so I'm free to cater to his every whim and desire, pampering him, tending to his every want and need, cooking/cleaning/sewing/personal shopping, and basically being whoever he needs me to be every moment.
I don't think I could fulfill my role he's asked me to fulfill if he didn't protect me financially. Never having to worry about if my bank card will swipe, or what would happen if he passed away, or how I'm going to ever financially survive in old age is an incredible gift. I feel so secure in my service to him.
he always drives behind me if we're following each other somewhere so he can monitor the traffic and be there in case of an emergency.
similarly I always walk just ahead of him so he can watch me, watch my surroundings. Unlike many Master/slave couples we know he'd go nuts if I walked behind him because he'd be constantly checking for my safety.
he legally conceal carries, practices gun safety, and requires that I have a working understanding of it as well.
he is continuously growing and learning about leadership and being in charge. It provides mdw comfort that he doesn't "know everything" and that he's always challenging himself.
he monitors my food, alcohol, and prescription drug intake. As a submissive with a naturally addictive personally type he'll approve or disapprove the above listed things to make sure I'm healthy physically and mentally. This includes making sure I'm taking vitamins, seeing doctors and dentists, and locking up addictive medicines so I don't have free access (bless this man.)
he opens all doors for me when we're together as a signal that he approves of where I'm headed to.
we practice full time power exchange including a decade now if orgasm control. He's written out end of life instructions for me to de-train/acclimate to being outside of his control if he passes before me.
I'm sure there's more. Here's an excerpt from a journal entry I've written that may explain more of this topic as it relates to us.
*When making a transition from D/s to M/s a transition in our wording happened too. I began using phrases like "I am owned" and "I am a possession" and "I belong to another person." I meant them.
When I tried to explain how we as a couple work, what it means for us to live this way, it became easiest to explain as if I were an inanimate object being handed for another to own. Easiest cause it's truest, of course.
My go-to became, "So... I'm like a beautiful piece of glass that has been handed into somebody else's care." It resonated with the person listening, allowed them to have a visual of our marriage. Sometimes "slave" creates a vision of a doormat and it couldn't be more different for us. Master would never step on me. I am delicate, and prized, and beautiful.
The more I used this example, the more it resonated with me.
I've become this object.
Nobody is more surprised than me.
I am a piece of glass.
I am delicate, and prized, and beautiful.
I wear my heart on my sleeve; I show my soul. I am transparent.
I am easily broken. I need protection. From my environment. From myself.
If I handed myself over to the wrong hands: I'd be easily mishandled and irrevocably destroyed.
In the right hands, the hands of a Master: I'd become something far better than I'd ever dreamt possible.
In the hands of a Master, over time and work and skilled practice, he's all of a sudden crafted me into a prism.
Mostly I'm still just me, a simple piece of glass, only sleeker than I was before being owned; rarer. I've been given to an Owner that keeps me close to him. Not just a possession but the very most special possession he has ever owned. He'd give his life to keep me safe. He protects me at all costs.
I am at my best existence when I do as is intended: when I silence myself in his presence, still myself until he moves me on his accord, and act as this object.
When I can do that, Master lifts me to the sun and uses his skill, his absolute knowledge of the thing he's created, and shifts me just so.
Only then can I reveal my real beauty.
My true colors can finally shine.*