I came to the conclusion that repetition is valuable by looking at how often, after giving a presentation in which I convey certain facts, the audience subsequently follows up in ways that make it clear that they neither retained the facts nor the awareness that I'd presented those facts. When I started making a point of repeating my key points several times during a presentation, tying it back to multiple different topics and multiple different questions, the incidence of that sort of followup question dropped.

That said, I haven't done a careful study, and I could easily be misattributing the result to the wrong cause. For that matter, I could easily be perceiving a result that isn't actually there. Humans make those sorts of errors all the time.

I agree that a lot of repetition is nervousness, and that a lot of it is an attempt to grab floor-time. (I'm not sure I'd call the latter nonstrategic.)

I also think a lot of repetition is an attempt to maintain control of the attention of the group. (As in:
A: "X"
B: "Y"
C: "NOT(Y)"
A: "X."

Looking back at this, it occurs to me that I may have misunderstood your question and thus answered a different one that you meant to ask. There are things that I take as real-time indications that what I've said has registered -- for example, being able to answer questions or to ask sensible ones -- and things that I take as indicators that it hasn't, such as asking questions I've already answered. When I'm talking to groups I often get neither, unless I've done enough prep to create exercises specifically intended to obtain them,