Quite often, people forget time needs to be spent to prepare the work done so others can see it. Even when everything seems to be done, a project is not finished. Marketing is part of it.
If you design something for yourself, you could stop when everything is done. But if there is a slight intention of sharing it with others –for example if you want someone to produce your design– more work has to be done in order to make your product understandable by others. That last bit, is marketing.
Marketing helps people to better understand what you do, it makes people want to buy what you create. If your product lacks quality, your marketing will try to lie to people, in some way, so they buy your product.
On the other side, when you have a high quality product, people will miss out if there is not enough marketing. They will not get to see how great your product is, they won't notice it.
Every product needs marketing no matter how good it is. Things that people need have to be marketed, but even more marketing is needed for things people do not need. That last type of marketing tries to induce people to want what you are selling.
Invest equally in the definition and documentation stage so that solutions can be normalized and compared, even if the solution is a failure. — Rober Fabricant
The documentation stage is marketing. It does not imply that you have to stop the design stage radically and then start making what you are doing understandable or prettier. It means that you should refine and prepare things to be actually in a finished state at some points. This will help to finish and to make all the small decissions that are usually taken minutes before a deadline, allowing yourself to judge what you have done, and how others will see it.
If after reading this post you still do not agree with the statement Everything Needs Marketing, a clear example I commented a while ago is how even sliced bread needed a huge amount of marketing to be the success that it is today. Even the ideas that thrive nowadays and seem so obvious that they had to be a success needed to be installed in everyone's brain, convincing them of how good they are.