Every minute of every day, someone needs blood. that blood can only come from a volunteer donor, a person like you who makes the choice to donate. there is no substitute for your donation. when you make a blood donation, you join a very select group. currently only 3 out of every 100 people in america donate blood. from its beginning, the american red cross has formed a community of service, of generous, strong and decent people bound by beliefs beyond themselves. the american red cross blood donor embodies this principle. join us in our mission to maintain a safe and stable blood supply by making your appointment to donate blood today.
based on this excerpt, what is the author’s primary purpose for writing?