Why does Charlie Marlowe begin his tale by invoking the past?

In the novel's first description of Marlow, his sallow skin and sunken cheeks do not portray him as healthy or happy. He has had the chance to explore, but apparently the experience has ruined him. This is Conrad's way of arranging the overall structure of the novella. The audience understands that this is to be a recollection, a tale that will account for Marlow's presently shaky, impenetrable state. The author is also presupposing knowledge of colonialism. The bitterness of Marlow's recollection suggests Conrad's strong bias against colonialism, which he seems to be imparting to the reader by expressing Marlow’s difficulties.