I always love how the subs rush to the fore to answer questions asked of dominants.
While all useful and welcome information as to circumstances and even methods, I heard in your question that you were curious what the dom would get out of it.
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The appeal of being capable of bringing out your girls' submissive side, of training them to cum, to be thinking of you throughout their day, to clearly need your approval, can be heady... satisfying... and is a great way to stay both aroused and in practice.
I have learned that there is a cadence to be found in chat, that can translate to voice skills in person. I can make my girl cum by voice command alone. Some come to it more easily, some never at all... but I have been successful in doing so in a number of instances.
They say the mind is the most erogenous zone you have. If so, then being able to communicate erotically is as important to your sub as being able to act erotically. So there is pleasure to be gained therein as well.