The colon: When the explanation is more important than what’s explained

Each punctuation mark serves a core function, and usage should follow the core function whenever the rules governing that function are applicable. Disputed Issues has considered the core functions of several punctuation marks: The comma sets off nonrestrictive elements; the semicolon neutrally connects independent clauses; and the dash emphasizes matter tangential in its immediate context. Following the core functions means eschewing rules unrelated to the core function unless the core function is unrelated to the construction. To take the comma, usage guides sometimes state the rule that a comma doesn't set off an adverbial clause at the end of a sentence, but the restrictive - nonrestrictive distinction the writer should apply eviscerates the rule.
The core function is the main function for ordinary discursive text. The colon has a variety of uses, such as exemplification by lists, but the central discursive use of the colon is to substitute for a word like because to create a clause more central than the independent clause to which it would be subordinated. From the opposite end of the grammatical telescope, the colon demotes the independent clause to a parenthetical role.
Here's an example:

Density is audience relative: the optimal density for experts is higher than for novices; but density's audience relativity isn't as great as you might think.

Grammatically, an adverbial clause could substitute for the clause following the colon:

Density is audience relative because the optimal density for experts is higher than for novices; but density's audience relativity isn't as great as you might think.

The colon serves better than the adverb, since the matter in the because clause is more important than what precedes, which only creates a transition through a more general proposition; the more important propositions shouldn't ordinarily be subordinated to the less important. The clause following the colon becomes independent when the colon is substituted, but this happenstance doesn't affect the colon's usage; a subordinate clause can follow the colon, and the independent clause's significance would remain parenthetical.