In this, the first of the series of visitations that Scrooge "suffers" during his Christmas season, the appearance of the ghost of Old Marley comes as something of a shock to Scrooge, especially because his fate since his death hints very strongly at the kind of fate that Scrooge can expect to see when he dies. When asked why spirits walk the earth, note how the ghost of Marley replies:
"It is required of every man," the Ghost returned, "that the spirit within him should walk abroad among his fellow-men, 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!"
The ghost is also chained by a chain consisting of "cash-boxes, keys, padlocks, ledgers, deeds, and heavy purses in steel," which, the ghost tells Scrooge, was "forged" during his lifetime, symbolizing the dominant place money had in his life and the way that he put its pursuit in front of other more altruistic causes. Of course, this is another aspect that he uses to frighten Scrooge, as he says:
"Or you would know," pursued the Ghost, "the weight and length of the strong coil you bear yourself? It was full as heavy and as long as this, seven Christmas Eves ago. You have laboured on it, since. It is a ponderous chain!"
Thus, since his death, the Ghost of Marley has been forced to wander the earth, bound by the chain he himself created during his life, to witness what he willingly forsook during his life and the situations that he could have improved and made happier. It also foreshadows the fate of Scrooge if he chooses not to change.