Because it so eloquently puts forth the goals of the Civil Rights Movement. It offered a window to the future, a vision of what American society could be in terms of race relations and social justice.
I think it has become even more important in the past few years given the election of the country's first African-American President, roughly 44 years after Martin Luther King's famous speech. On Inauguration Day, I was in Washington DC, and once the crowds had dispersed (which took forever), I walked from the spot in front of the Washington Monument where MLK had given that very speech, to the front of the Capitol Building where Obama had been sworn in. It took me about an hour. It took MLK and the nation almost half a century.
Dr. King's speech has historical relevance because it literally gave a voice to the Civil Rights Movement. In one speech, he was able to give structure and form to the movement. The speech reached across all boundaries and humanized the struggle as not one rooted in political goals as much as human decency. The Civil Rights Movement was given a structure that expressed how the denial of such rights is tantamount to denying human dignity. The speech was also a way to pull more people into the struggle, weakening resistance due to apathy or a lack of awareness. The speech was important historically because it helped to match a specific voice to the overall movement.
This speech was important historically for a few reasons:
1. It symbolizes a moment in time, the Civil Rights Movement. As stated above, this was important to secure equal rights regardless of race.
2. It references major pieces of history attaching them to the moment he was creating himself. He quoted from the Declaration of Independence, alludes to the Gettysburg address and Abe Lincoln, and references the music of both a Negro spiritual and a national patriotic song, My Country Tis of Thee. Each of these pieces deal with freedom and the rights of man.
3. He united the country by citing many mountain ranges in many states. This is important because each section of America represents mini-cultures.
4. The inspiration that this speech induced models what happens in all great movements. Thus, when people want to cause a great act, here is a model worth a good look to copy. King is noted for his non-violent approach to enacting change and this speech symbolizes that.
The reason that this speech is considered to be an important part of history is that it was connected to the 1963 March on Washington and that march is connected to the passage of the Civil Rights Act of 1964. That law was the most important law in terms of the rights of minorities to be passed in the 20th century, in my opinion.
What the Civil Rights Act said was that no businesses or public institutions (except for private clubs) could discriminate against people on the basis of race and some other things. This meant that non-whites could no longer be told to sit in the back of a bus or movie theater. They could no longer be banned from certain restaurants. This was hugely important.
Well its a lot more important now than it was before. It really became famous post-mortem.