Rhetoric can be an important way to track meaning in literary works. It is all the more important in understanding the meaning of The Bet (despite our version being translated from Russian) because so much of the story focuses directly on language and reading. The lawyer, in particular, primarily lives in the worlds of books during his years of solitude. Therefore, it stands to reason that we could learn something about his character from examining his use of language.
With that in mind, consider these samples from the lawyer's last letter to the banker (emphasis added in both quotes).
"You have lost you reason and taken the wrong path. You have taken lies for truth, and hideousness for beauty. You would marvel if, owing to strange events..." (16)
"I have climbed to the peaks... I have seen the sun rise ... I have watched from there the lightning flashing ... I have seen green forests..." (15)
The repetitive phrases beginning each sentences constitute a case of the rhetoric device called anaphora, beginning successive sentences with the same words. This device serves to emphasize the lawyer's point. He is writing this letter to the banker to explain the action that he will take tomorrow, and why he is escaping early. He is using this rhetorical tactic to drive home how sure he is of his choice, and everything that he has experienced.
But of course, it isn't enough to merely notice anaphora: to truly grasp its value, we need to connect it to the broader narrative, especially considering our initial observations about the lawyer's intimate relationship with language during his imprisonment. Given the sheer volume of writing the lawyer absorbed, he presumably could have written in any style he liked. What does it say about him that he chose this sentence structure in particular?
Well, if we think that language is linked to thought (which seems intuitive enough), we can glean two plausible insights about the lawyer from the language he uses here. First: anaphora renders groups of sentences very simple because they all have the same overall structure. This suggests that the high level of understanding he has achieved from all of his studies have allowed him to see the world so clearly that he can recognize complex patterns and phenomena as simple interrelations between things. Second: even if he wanted to express things more complexly, he probably wouldn't do so because other people, like the banker, wouldn't be able to understand him. Thus his language also reinforces what he says in the letter: he thinks other people too horribly confused in their understanding of the world to even be able to hold a conversation with him.