Chekhov masterfully uses flashback in order to condense time and filter out details that do not focus upon the psychological development of the main characters and the theme.
Often when reading a novel, readers turn back to earlier chapters and re-read them because the relevance of some passages has deepened as the narrative develops. In shorter works of fiction, authors often use flashbacks as tools to help people "re-read" what needs to be understood, as well as to filter out non-essential details. Flashbacks benefit a narrative when knowledge of conditions or actions from the past can be used to provide significance to present conditions in the narrative or to underscore theme.
In his story "The Bet," Anton Chekhov illustrates the vanity of the desires of man. The young lawyer believed that life in isolation would be no real sacrifice and he would profit greatly with his reward at the end of his term. But what he really learned is that without human contact, such things as books, music, writing, languages, theology, and history are of little value without anyone with whom to exchange ideas and share the emotions that these studies evoke.
Through the use of flashback, the older and wiser banker looks back upon the time of the bet and he realizes,
On my part it was the caprice of a pampered man, and on his part, simple greed for money...
In a similar fashion, the letter written by the lawyer who has spent fifteen years in isolation explores his wisdom gained from experience as he looks backward. After an impulsive wager affected so much of their lives, both of these men now realize how shallow and vain their thinking has been all these years.