People who have been living in the U.S. illegally for over a year and then leave the country can seek legal residency if they don't re-enter the U.S. for at least 10 years. However, many people have family obligations in this country that would make deportation and a decade-long re-entry ban very difficult for their loved ones.
You've gone through the long, often-frustrating process of getting a green card. Perhaps you plan to become a U.S. citizen one day. Maybe you just want to be able to live and work in this country for the foreseeable future. Either way, your green card status is valuable, and you don't want to do anything to risk losing it.