Back in the dark ages when I took physics, I learned about the six simple machines. When dealing with marketing, I find the lever makes a good analogy. It increases the force applied by a given amount of effort. Rephrased: You don't have to work as hard to get the same result.
Call me lazy, but any time I can get the same result with less effort - especially in marketing - I'm all over it.
The concept is simple. In marketing, we need to apply effort to gain purchase - literal as well as figurative. The more leverage we have, the less effort needed.
Marketing isn't the lever. It's the fulcrum. It's base you need in order to focus the application of effort. The fulcrum needs to be solid enough to take the load of both the effort and the output. If it's too squishy, applying effort will crush it. It can't really be too strong, but there's no need to make one stronger than you need. A fulcrum that can support ten tons doesn't actually give you any advantage when you're only trying to move a ten pound load.
In this context, marketing is not what you do. It's the way you've decided to do it.
I write SF/F novels. I self-publish them. I use social media as my primary communication channels. I follow the "Big Frog, Small Pond" and "1000 True Fans" strategies. Those were my decisions regarding marketing. That's my foundation, my fulcrum.
Effort is the sum of all the forces applied to the lever. For authors, that can be time spent at conventions, on social media, or writing blog posts. It can be cash spent on ad buys and market research. It can be anything the author does to promote the works. It's built from all the decisions an author makes about strategy and model. What do you have to offer? Where do you offer it? How do you choose to make people aware of your product?
The output load in this case is the number of purchases or - perhaps more accurately - profit. After all, it does little good to spend $10 for every $1 in revenue. It might be advantageous to achieve some short-term goal, but it's a bankrupt model in the long run. Literally.
The lever is the key. The lever is what multiplies the effort and provides the applied force to the output. For authors, that's the backlist. If you only have one book to sell, then the lever is short. You have to apply a lot of effort to get a unit of output. If you have two books to sell, then you get a multiplier. Perhaps people who buy the first will buy the second. You have more visibility - a bigger footprint - which makes your lever longer but also stronger. Add a third and a fourth and a fifth and you begin to build a machine where only the lightest touch of effort can give you a huge amount of purchase.
It's just simple physics.