The overflow is specifically to allow excessive fuel, from leaking intake fail, to bleed out safely (well from not depositing into engine etc) away from other parts of the bike unimpeded.
You wouldn't want the carbs to suck air from that tube ideally, possibly water etc has access.
Vent tubes allow "safe" ingress of air from discreet source, in or out, to make sure there's no pressure plus or negative allowed to accumulate in float bowl, making them as near neutral as possible so that the vacuum through the carb venturi and pulled through the jets is absolutely as repeatable as possible to give consistency in jetting/mixture.