Why is "I Have a Dream" considered the defining moment of the American civil rights movement?

The timing, content, setting, and legacy of Dr. King's iconic speech mark its delivery as a defining moment of the American Civil Rights Movement.
Dr. King gave the speech at the foot of the Lincoln Memorial in America's capital in August of 1963 during the March on Washington for Jobs and Freedom. A quarter of a million people gathered for a nonviolent protest of social conditions for African Americans, and the setting was symbolic. President Abraham Lincoln had signed the Emancipation Proclamation one hundred years earlier.
Dr. King's thesis was that the promise of the Emancipation Proclamation had not been fulfilled, because African Americans had still not received their full rights as American citizens.
Less than a year later, President John F. Kennedy proposed the Civil Rights Act, which banned discrimination based on race, color, religion, national origin, or gender. Dr. King's speech paved the way for moving that legislation along. The law was designed to end manipulation of voting requirements that kept African Americans from the polls and to desegregate motels, hotels, schools, and so on. It also outlawed discrimination against African American job seekers.