Hi There,
Let me try to explain him.
If I take an Example to explain you. When we make a building, we need a design/Blueprint/Architectural plan then we make the pillars, wall etc.
Similarly to develop a website we have to decide how many pages you want in a website and suppose in Homepage where you have to put LOGO, Social media buttons, where you have to place content, How many images you want to add and where you have to place that images.
This will help you to save your time, Money and effort.
Hope you got my point.
Adrian