The Us green card rush exists because the card entitles one to permanently live and work in the USA. It is an official ID document. The popular immigrant visa gives the Card owner almost all the rights of a US citizen.
Benefits of winning the Green card
The holder of the card enjoys the same rights as any American citizen, with the exception of the right to vote and that of being a juror in a trial.
On winning the visa one has access to health coverage.
Social security also is accessible to card winners.
Retirement benefits is another crucial benefit that befalls a card holder.
Having a Green card also allows you to apply for naturalization after 5 years.
One can renew the permanent residence card upon request, Thus, it is possible to keep the card for life if you meet certain conditions (declaring your taxes, paying your taxes in the United States, not leaving the United States for a long period, etc).
If you win the card you can easily move in out of the US in comparison to having a temporary visa or student visa.