Why add lupin flour to white bread?
From a seller's product description1:


  In der Backindustrie verwendet man Lupinenmehl als Zusatz zu Brotmischungen, da es das Brot aromatisiert, elastischer und länger haltbar macht.
  (The baking industry uses lupin flour as additive in bread mixes because it makes the bread more aromatic, elastic and increases shelf life.)


Another description2:


  Das Mehl kann in Brot, Kleingebäck und Teigwaren verarbeitet werden. Lupinen-Mehl bindet relativ viel Wasser.
  (The flour can be used in bread, pastry and pasta. Lupin flour binds comparatively much water.)


So it boils down to lupine flour being used as a dough enhancer, the content in your example being 1.5% or less of the total bread weight suggests that it probably is not really "tasteable", but influences either the manufacturing process or the texture of the bread. If lupine flour is used to alter the protein / carbs ratio, up to 15% of the flour is replaced by lupine.

So my guess is that the manufacturer benefits by being able to incorporate a tad more water (which is sold, too) and possibly a bit less gluten formation, making the dough easier to handle and giving it the "fluffiness" that a German customer expects from what is labeled as "American Sandwich" bread.