Certified GKH Gardening Expert
They complete photosynthesis. They are leaves, like others, just growing in a different spot, higher on the plant.
In order to get seeds, you must allow the plant to live into its second year, in which, it will bolt, or go to seed.
We must remember, that most of these Brassicas are HIGHLY modified, by humans. This plant doesn't resemble the original from where it came, in Rome.