It is a temporal or topical "marker" within the text.
A narrative milestone is a trigger and a marker and it marks not just the passage and unfolding of information but also the sparking of emotional connection which keeps the reader engaged.
It helps the reader or the listener develop categories or patterns for the creation of schema (or schemata) that will search as a framework for organizing information. It also helps the reader stay "hooked" or engaged in the text.
There should be a milestone at the beginning of the text. It does not have to be the same as a topic sentence, but should definitely communicate how/why a listener should care about what is being said. Then, there should be milestones at regular intervals within your text. If you're presenting it verbally or via video conference, include a mini-milestone every 30 to 45 seconds, and a major one every 2 minutes.
An effective milestone is a great "hook" and combines conveys important facts while sparking an emotional connection.
No. A milestone can trigger emotions, and so are effective rhetorically in utilizing both pathos (emotions) and logos (logic/facts). Thus milestones can be effective in a persuasive, emotionally compelling document or presentation.