The best analysis of Martin Luther King Jr.'s purpose in using the imagery of valleys, hills, rough places, and crooked places in this passage is:
King uses the imagery to symbolize the removal of societal inequalities, suggesting that the future holds a just society where barriers to equality and justice are eradicated.
This interpretation captures the essence of King's vision for a future in which obstacles to equality, both literal and metaphorical, are addressed, leading to a society where justice is achievable for all.