simplest version;
The submissive is there to service the Dominant-- not the other way around. The dominant might get domestic caretaking, such as chauffeur and housekeeping services, and personal services such as massages and blowjobs on demand. The submissive gets pleasure and satisfaction from performing these roles and making their Dom happy.
I accept this, but I'm interested in the relationship between s/m and dom/sub. People have said already that in s/m you usually have a Dom acting as sadist and a Sub acting as masochist. If the Sub's purpose is to service the Dom, how does the Sub as masochist service the Dom? Does the Dom have to be acting Sadist for the relationship to work?