Martin Luther King Jr.'s main purpose in delivering his "I Have a Dream" speech was to obtain equality and equal rights for all people. During his speech, Dr. King alludes to many previous times in history where equal rights have been promised, but the promises have not been fulfilled.
"...but one hundred years later, the Negro still is not free."
The purpose of Dr. King's speech was to show society that this would be the end of the long list of failed attempts at obtaining freedom.