I do this too, all the time! I agree that those 2 scenarios are common (especially at university), but I feel like there is another case in which nodding along helps conversations move more quickly.

For instance if somebody mentions “nuclear nanoprobes” it might just be a casual remark on the way to a more important point. It may be that I will understand the following point with no knowledge of “nuclear nanoprobes”, in the case where this doesn’t hold one can just ask for more info.

I think of this as lazy evaluation for human beings ;)