This is known as an emulsion tube in Weber parlance. It is designed to introduce air from the main air bleed into the fuel from the main jet and mix them into a fuel-air emulsion and that is what is actually feed to the venturis in the throat. The number of holes and the position of the holes in the tube will vary how much air is introduced into the fuel-air emulsion at different air volumes/vacuum levels in the throat, and so fine tunes the main jet operation as air through the throat increases (RPM increases).
The emulsion tube is fixed in this carb design, but in many Webers and Dellortos, you can change the emulsion tubes and there are are whole families of emulsion tubes with different air hole patterns that will make the mains in the carb 'come on' at different RPM's and over certain RPM ranges. It has a pretty strong effect on how the carbs perform, and you can cure transient lean or overrich conditions by tuning these tubes. It can get VERY tedious to juggle the emulsion tubes along with the main fuel and air jets all at the same time. (I've done it twice and it takes a lot of plug reads and seat-of-the pants runs to get it right, but it sure makes the engine run and respond well.)
I will venture to guess that in this carb, perhaps the bottom hole is plugged or left open depending on the engine and application.
