Since the green card process is so unpredictable (not only are there delays, there are denials as well), a lot of people go on with their lives, keeping their jobs, signing up employment/lease contracts, having babies, and such. Many of you have been asking that once the application for permanent residence in the United States is approved, how much time the immigrant has to arrive in the United States.
You see the goal of obtaining permanent resident status in the USA is to live here permanently and establish strong ties to the country. So it is expected that once you are approved, you will do exactly that, and in order to allow people to uproot themselves from their lives and start a new one in America, they have up to six months to officially enter the country. While the GC does allow extended travel abroad (up to 6 months) and even stays longer than one year with proper permissions, if the immigration officer suspects that you really do not live in the US, actually live overseas, and lack ties to the US, the green card can be revoked.