How did Marley explain to Scrooge why spirits walk the earth?

Answer 1

The original quote is "It is required of every man," the Ghost returned, "that the spirit within him should walk abroad among his fellowmen, and travel far and wide; and if that spirit goes not forth in life, it is condemned to do so after death. It is doomed to wander through the world-oh, woe is me!-and witness what it cannot share, but might have shared on earth, and turned to happiness!"

Answer 2

to protect there loved ones from danger and warn them of disaster