One of the most magical elements of A Christmas Carol is that memories are totally accessible. Scrooge doesn't have to remember them—he lives them, which means his memories aren't tainted by, well, other memories and lessons learned (or unlearned). With the right guide, Scrooge is able to examine and draw conclusions from specifically those memories that are most relevant to the problems he faces in real time. Convenient! At the same time, these same memories are used to humanize and explain the otherwise monstrous and almost inhuman Scrooge—to make readers sympathize with him rather writing him off as an irredeemable jerk.
Reliving his memories is what causes Scrooge to finally connect with his senses of smell, touch, taste, and hearing—rather than shutting them out and distrusting them like he does when faced with the ghost of Marley.
Instead of simply explaining Scrooge, his memories attempt to excuse him, as if to say, hey, he had a bad childhood, so cut the man some slack.