The title "Dusk" seems quite appropriate for Saki's story for several reasons. Obviously, it creates a mood associated with the setting. Most importantly, however, is the fact that dusk is the time when certain types of criminals are most likely to come out to prey on the public. These would include prostitutes as well as con artists like the young man who claims to have lost his hotel.
The time was early in the twentieth century. London streets were patrolled on foot by policemen well acquainted with their particular beats and capable of spotting any irregularities, which would include people soliciting others. Dusk provided a protective cover for people like the young con man.
Dusk also lent credibility to his story. He badly needed a roof over his head before nightfall or he might have to sleep on the Thames embankment. It would be a very cold and dangerous place to sleep. Hopefully, this fact would help to elicit sympathy and help to make the young man appear to be a respectable citizen who was not accustomed to asking strangers for money but was forced to do so because of his increasingly desperate circumstances.
Saki opens his story with a description of the setting from Gortsby's point of view.
Dusk, to his mind, was the hour of the defeated. Men and wowmen, who had fought and lost, who hid their fallen fortunes and dead hopes as far as possible from the scrutiny of the curious, came forth in this hour of gloaming, when their shabby clothes and bowed shoulders and unhappy eyes might pass unnoticed, or, at any rate, unrecognized.
Gortsby has money in his pocket and is sitting fully at ease. He does not feel particularly sorry for these defeated men and women. He is
. . . not disinclined to take a certain cynical pleasure in observing and labelling his fellow wanderers as they went their ways in the dark stretches between the lamp-lights.
This description of the unhappy wanderers at dusk and of Gortsby's cynical attitude are most important to the story. He is going to undergo a transformation when he finds the cake of soap by the bench and naturally assumes the young con man dropped it. Gortsby suddenly feels guilt, shame, and remorse as he hurries after the young stranger to lend him the money he needs for a room. No doubt Gortsby is castigating himself for his cynicism and cruelty, vowing to change his attitude towards suffering humanity as a result of this lesson he has learned. He actually says to himself:
"It's a lesson to me not to be too clever in judging by circumstances."
Then as he is returning to the vicinity of the park bench, he sees an elderly gentleman searching for something and learns that the cake of soap belonged to him. This proves that Gortsby was right in being cynical and selfish in the first place. The elderrly gentleman who says he lost a cake of soap may be a con man himself. He may have left it there on purpose, intending to come back and use it as an excuse for starting a conversation with Gortsby, leading up to the same story about having lost his hotel and needing a loan to get a room somewhere for the night.
Gortsby's experience dramatizes Saki's philosophy that this is a cold, cruel world in which you can't trust anybody. Saki was described in a Wikipedia article as a Tory and something of a reactionary. Early in "Dusk" he describes the wanderers in the dusk as "men and women, who had fought and lost." From his perspective, life is a struggle of all against all, of "survival of the fittest."