The role of the young man, in Saki's short story "Dusk," is to prove Norman Gortsby's thoughts on those who come out in the dusk. Norman believes that
Dusk, to his mind, was the hour of the defeated.
Essentially, Norman is out at dusk. While readers may not immediately consider that Norman may be one of the defeated, the young man proves that he is.
In order for one to be defeated, another person must succeed. The young man is able to get money out of Norman, the one thing that he seems to be out looking for (given his story). The young man, upon approaching the bench, may have seen the soap. The soap, then, gives him a perfect ally for his story. Norman can only believe the story after finding the soap. In the end, the soap did not belong to the young man. It was simply used to get the young man what he desired.
The young man's role is explicit. He serves as the catalyst which proves Norman's thoughts on those who come out at dusk.