If absolutes about writing exist, this is definitely one:
audience analysis determines everything about a piece of writing,
from what you say to how you say it.

Believe it:

There are no rules; the audience rules.

This lost quotation from a University of Maryland University College (now UMGC) webpage is partly available in this video. The idea it gets at is that effective writers in most cases identify and then write to a specific audience.

When we begin a writing task, we often focus on the question without considering the audience who will read our answer. But our audience matters, whether we are writing a business plan for a client, a diary entry for our future self, an academic paper for a course, or a personal statement for a grad school application.

Effective writers consider their intended/immediate audience (and who else might read their work). As the quotation above suggests, attention to an audience can determine everything: purpose, argument, genre, organization, sourcing and citation format, and writing style.

Audience Matters: this UNC-Chapel Hill Writing Center page offers tips on identifying and writing for an audience beyond your professor.

Adapting to Your Audience: this Colorado State University Writing Studio page offers comprehensive advice as you evaluate and write for different audiences.