The first function of the bracts around the base of an aster flower head is to just hold everything together. But why pass up opportunities. They are also used for other purposes. Here's a tuberous Indian-plantain with showy white involucres. The flowers are reduced and play at most a small role in attracting attention. You might see this plant in large patches where the flat inflorescences make a very attractive sight. These will catch your eye before the flowers are out, and even after.