What is the function of these 'bulges' on the frame? A friend brought me an old frame that he found on the roadside on bulky waste day (is that a thing outside of Austria and Germany?).
The rear wheel was gone, the front one bent beyond repair, but the rest of the bike was in quite good condition. From the parts on it (a complete Shimano 105, 2x7 groupset) I guess it's from the early 90ies.

The interesting feature on it, one that I have never seen before, are these "bulges" on the main tubes (top tube, down tube, and seat tube). 

The feature is visible on the second photo, and in the third image I have made a crude sketch of the cross-section of those tubes.

Does anyone know WHY they are there? Is there some engineering idea behind that? Is it just an optical feature? 

PS: I thought that maybe it was some kind of identifying feature of the manufacturer, but I can not find any information about a bicycle manufacturer "Oberriet" (see photo nr. 4) at all. This just made me even more curious.
My best guess is that you have a frame made of Gilco Design Columbus tubing used on Italian bikes in the 80s. I could find no mention of Oberriet bike brand anywhere. Maybe a small custom frame builder that didn't get much (any?) coverage.

This frame tubing looks a lot like yours


In researching Colnago came up often as a bike maker that used shaped tubing.  


  In response to criticism that his frames were not stiff enough, next Colnago experimented with ways to change the behavior of frame components. In 1983, he introduced the Oval CX with an oval-shaped top tube to add stiffness. He then experimented with various crimped-tube frames which became production models as their top of the range frames, beginning with the "Master." Later "Master-Light", Master Olympic and Master Piu extended the range. Colnago built a frame from Columbus tubing used by Giuseppe Saronni to win the world professional road race championship in 1982, and afterwards a short-lived collection of bikes were badged with the Saronni name. In 1983, Giuseppe Saronni would go on to win the Giro d'Italia stage race on a Colnago bicycle wikipedia  


Later in the same article there is a list of steel frame bikes with descriptions of the tubing along with production years. From your drawing the tubing has six ribs which could be a tube set used on the Regal 1986.

I don't think your bike is a Colnago, I think it is a bike made from a similar Columbus tubing set. The answer to the "why did they make it this way" is - as mikes said in comments - stiffness.

Even today the Colnago Master is made with shaped steel tubing.