Do "overview" sections increase learning outcome?
There is something called "advance organizer", that traces back to Ausubel (1960). It is some kind of overview, in which a short summary is given and (more important), in which the following  topic is connected to prior knowledge (= Associations are being activated). In that sense it activates schemes, in order to assimiliate the new knoweldge to the old one or to accomodate/correct the prior knowledge. So, to be useful, advance organizers should have certain features:


Connections between the following topic and prior knowlege
Catch the attention of the student
Be concret, insted of abstract


F.e. Before teaching equations the teacher could give the analogy of a scale. 

From a scientific perspective it is known, that advance organizers can have supportive effects on learning and retention (see f.e. Luiten, Ames &amp; Ackerson, 1980)

Luiten, J., Ames, W., &amp; Ackerson, G. (1980). A meta-analysis of the effects of advance organizers on learning and retention. American Educational Research Journal, 17(2), 211-218.