What is the function of bracts?

When a flower arises in the axil of a leaf-like structure, the structure is called a bract.

A bract is a modified or specialized leaf, especially one associated with a reproductive structure such as a flower, inflorescence axis, or cone scale. Bracts are often (but not always) different from foliage leaves. They may be smaller, larger, or of a different color, shape, or texture.

They can be green as seen in ordinary leaves or can be colored. Since they are large and brightly colored, they are often mistaken to be petals. This causes insects to get attracted to pollination either together with the perianth or instead of it.