Writing is one kind of interaction, one kind of meeting between parties. And so, there are also conventions to writing. Conventions that make people comfortable. Make people feel that they know what is going on when they read what you have written. Black text on a white background is a convention.
Margins are a convention.
Meaningfully placed white space on a page, space that blocks off, separates and focuses the reader's attention. That is a convention. There are large numbers of these conventions and they will be discussed in some detail in later issues of Uncommonly Clear . All these conventions taken together might be called literacy . They are what makes a person understand what they have read.