Those holes aerate the fuel and help lift it up to the throat of the carb (at the top of the emulsifier tube). When the engine is running, this tube is immersed in fuel that was pushed up from the float bowl, because the air pressure in the float bowl is still and at higher pressure than the moving air in the carb throat. These little holes "bubble" air into the fuel column, air which is supplied from the bell-shaped area at the back of the carb: this air is at about 10-40% higher pressure than the air in the throat, but 90-60% of the pressure in the float bowl (depending on throttle opening). Thus...then the fuel gets pushed up the emulsifer tube by the bowl, it gets bubbled (aerated) by the air entering these little holes, so it can arrive at the mainjet in an almost steam-like state for the engine to suck up.
The holes nearest the top of these tubes aerate the low-speed operation. As the throttle opens more, the fuel level in this tybe drops accordingly, being limited by the mainjet, and aerates it about the same amount because fewer holes are then "under water", so to speak. At full throttle, this tube is nothing but bubbles of air in fast-moving fuel.
...and now you know how these carbs work! The idle circuit is identical, just smaller.