The need for organ donation is great. Currently the organ donation waiting list (as cited by organ donor.org) is comprised of 107,816 candidates. I think this in itself defines the need for donors. Currently the liver, kidneys, heart, lungs, pancreas, small intestine, and eyes are capable of being donated. If one person signed up to be an organ donator think of all the people they could help.
There is a great need for minority donators. This is due to the high prevelance of particular diseases found in certain ethinc groups.
"For example, African Americans, Asians and Pacific Islanders, and Hispanics are three times more likely than Whites to suffer from end-stage renal (kidney) disease, often as the result of high blood pressure and other conditions that can damage the kidneys. Native Americans are four times more likely than Whites to suffer from diabetes. "
Each organ and tissue donor saves or improves the lives of as many as 50 people. Giving the "Gift of Life" may lighten the grief of the donor's own family. Many donor families say that knowing other lives have been saved helps them cope with their tragic loss.
Organs, tissues, stem cells, and blood can all be donated and can save or improve many lives. Many times people need these things in order to continue living. Organ donation does not only come from people who have died. Living people can donate a kidney, blood, bone marrow, etc.
So are you asking why it is that there is a need for organs to be donated?
If that is what you are asking the reason is because there are a number of conditions that make it more or less necessary for people to have new organs put into their bodies. In some of these cases, the need for the new organ is complete -- the person would die without one. This is true for things like heart transplants. So, since people sometimes come to need replacement organs, there is a need for organ donation so long as artificial organs cannot be reliably made.