On matching type to purpose.
Suppose I am typing and wish to emphasize a word. Should I make it italic or bold?
The answer depends entirely on how I want it emphasized. If my goal is to make the word stand out on the page to aid in skimming the text then I should go with bold. Bold is used in technical works to identify words that are being defined, for example, so that when a reader wishes to recall the definition a simple glance suffices. If, on the other hand, I wish to add emphasis without interrupting the flow then I should go with italics. Novels, for example, almost always use italics instead of bold because they want the text to be accessed in a linear fashion with no distracting out-of-sequence cues.
This simple case reveals a question that a typographer must ask in many situations. Margin notes, footnotes, end notes, or parentheticals? Justified or ragged margins? Quotation marks, alternate typefaces, indented passages, italics, or just context? Etc. Each of these decisions impacts flow, accessibility, scannability, and the hard-to-describe flavor of a text.
One of the common signs of an unskilled typeographer is the use of typographic devices with no evident thought to purpose and impact. Conversely, one of common characteristics of truly beautiful typography is the judicious use of out-of-pattern typographic devices to convey nuanced mood. As with so much of art, the rules must be known to be well violated.
I claim no particular refinement in my typographic sensibilities. For me, the simple question “good flow or ease of reference?” is about the extent of my practical decision making. How oft I wish others had asked even this!