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.
Our airways close up to make them more efficient. The respiratory tree from the mouth to the terminal bronchioles can't absorb oxygen, so each time you breath all the air moved into and out of the trachae, bronchi and terminal bronchioles is extra unnecessary work for the muscles. We can make it more efficient by shrinking the size of those tubes down by contracting the smooth muscle.
When you exercise these small tubes won't be large enough to get enough air in and out, so the smooth muscle relaxes to allow more air to move into the lung faster. It's a bit less efficient, but necessary when you need a lot of oxygen.