Is there any reason behind fancy bricks in invisible places? In set 60080 there is a blue bick with pretty fancy shape.  There are also hardly visible bricks in other places. Is there any reason? Why are these bricks used? Any official or semi-official explanation? 

As far as I remember back when I was young, such pieces were rare, and (if included at all) were used in alternative models proposed on the back of the box. But this set has no such alternative models shown.
This piece is only two plates high, to have another piece in here you would need to stack two plates, which would  


make the total pieces count rise, or more expensive
might be more difficult to tear down by hand
probably less sturdy (with a piece on it acting as a lever stacked plates could pry apart)


Sets designers are under constraints for both piece count, piece prices, 
ease of setup/teardown and sturdyness so I guess this arrangement was decided. 

Some examples of other sets with the same kind of piece for the same kind of purpose:

set 4435-1



set 60108-1



set 60148-1