Good design is often coherent. It follows internal rules. People are comforted by order and rules. They think rule-based design looks “neat”. Their eyes understand the rules the design follows. They know where to look next. Even if they don’t realise it.
It’s easier to make good design if you set rules. And, importantly, follow them. When you set visual design rules it is often called a “system”.
A well-known visual design system is the layout grid. The page or screen is split into boxes that you fill with content. A difficult problem becomes a simple one. You pick up a piece of content. You decide which boxes to put it in. You have fewer choices so they’re easier to make. You can focus your energy on creative decisions. You don’t have to think about basic problems like alignment.
But grids aren’t the only useful design system.
You can create rules for your typography. Type scales for example. You’re only allowed to pick from a small set of text sizes.
Your rules don’t have to have a good reason to exist. Arbitrary constraints are “just because” rules. Maybe you’re only allowed to use one colour. Or one typeface. You have less options. But you still want the design to look good. So you have to be more creative. To think of ways to make the design interesting.
Arbitrary constraints make you a better designer. They teach you how to do more with less. To be more effective in a simpler design.
Limit yourself on purpose. You will be a better designer.
The consistency of a design is provided by the appropriate relationship of the various syntactical elements of the project: how type relates to grids and images from page to page throughout the whole project. Or, how type sizes relate to each other. Or, how pictures relate to each other and how the parts relate to the whole.