Having muscle tissue in our bronchioles that can constrict seems like a poor choice for tissue. Why would our airway want to ever close up? Wouldn't it be more beneficial for our bronchioles to just remain open?
There are at least two things to consider.
First, ability to limit airflow is a defense mechanism for animal. Imagine getting into area of some sort of toxic evaporation, e.g. CO2 cloud near volcano , then it makes sense to decrease delivery of toxin via lungs to minimum. As I understand, that is what an allergic asthma attack. (Sorry for not providing good enough source of that)
Secondly, you are incorrect in assuming that normal state is "dilated". Dilation of branchioles is sympathetic ("fight-and-fly") response of the nervous system to something like danger, that requires short-term boost in energy production. That is, by default, your airflow is limited. Probably, to limit amount of energy you effectively burn via oxygenation. But most importantly, you leave yourself a reserve in terms of oxygen supply for critical moments.
Some more information you might find here.
We don't really know. Several theories have been put forward over the years. Here are a few:
- contractions to assist exhalation or mucus propulsion
- promoting lymphatic and venous flow
- ventilation/perfusion matching
- stabilizing airways
- enhancing the effectiveness of cough
- optimizing anatomic dead space volume
None of them have a huge amount of evidence or support. It's very possible it's vestigial, like the male nipples or wisdom teeth.
The most popular is that it can limit airflow as a defence mechanism. This makes no sense to me at all - you don't need smooth muscle to limit airflow. You can just breath less. Ultimately airflow needs to match your rate of respiration or you'll suffocate, this fact doesn't change when there are toxins in the air.