To understand the intent of the phrase "bread and butter" which may not be known as an expressive idiom by those outside of the US is defined as an adjective
The meaning(s) are similar but can have different connations for each person.
https://www.dictionary.com/browse/bread-and-butterproviding a livelihood or basic source of income; supplying the basic needs of life:
i.e. a bread-and-butter job; the agency's bread-and-butter account.
of or relating to basic needs:
i.e. housing and other bread-and-butter political issues.
basic or everyday; staple; routine.
expressing thanks for hospitality:
i.e. a bread-and-butter letter.
I am going to borrow from "supplying the basic needs of life", "basic or everyday; staple; routine", and "expressing thanks for hospitality"
In a Lifestyle arrangement (24/7), both a Dominant and a submissive to supply each other with the "basic needs of life" which is often established through "basic or everyday routine" of the day to day life. The Dominant may establish chores, tasks, expectations, and other daily life activities in the form of cooking, cleaning, reading, continuing education. The Dominant offers this structured routine that can benefit the submissive in having daily goals that yield and constant for them in the form of routine.
The submissive, receiving the structure and purposeful goals will most likely be happy for the day to day routine of life. Too much spontaneity can cause chaos for everyone.
Although, spontaneity is equally important for both parties, as it does break up the routine and can evolve to nice surprises like that butter slowly melting on bread. This can be in the form of physical objects, surprise pleasant session play, cuddle time for the joy of it, and other forms of a Dominant ensuring their submissive is greatly appreciated by them. It creates the constant full circle of a symbiotic relationship dynamic of both parties to further allow the continual enjoyment of company.
It is this "bread and butter" of both "routine" and "expressing thanks for hospitality" that forges the bonds of a Lifestyle Dominant and submissive in my personal view.