"Well, it was this way," returned Mr. Enfield: "I was coming
home from some place at the end of the world, about three o'clock
of a black winter morning, and my way lay through a part of town
where there was literally nothing to be seen but lamps. Street
after street and all the folks asleep--street after street, all
lighted up as if for a procession and all as empty as a church--
till at last I got into that state of mind when a man listens and
listens and begins to long for the sight of a policeman. All at
once, I saw two figures: one a little man who was stumping along
eastward at a good walk, and the other a girl of maybe eight or
ten who was running as hard as she was able down a cross street.
Well, sir, the two ran into one another naturally enough at the
corner; and then came the horrible part of the thing; for the man
trampled calmly over the child's body and left her screaming on
the ground. It sounds nothing to hear, but it was hellish to see.
It wasn't like a man; it was like some damned Juggernaut. I gave
a few halloa, took to my heels, collared my gentleman, and brought
him back to where there was already quite a group about the
screaming child. He was perfectly cool and made no resistance,
but gave me one look, so ugly that it brought out the sweat on me
like running. The people who had turned out were the girl's own
family; and pretty soon, the doctor, for whom she had been sent
put in his appearance. Well, the child was not much the worse,
more frightened, according to the Sawbones; and there you might
have supposed would be an end to it. But there was one curious
circumstance. I had taken a loathing to my gentleman at first
sight." (Chapter 1)
As you can see from this snippet there's a story afoot that paves the way for the rest of the novel. I don't think Stevenson used this story to produce tension because it's simply a story between two men, a memory of a happening, but I do believe this is an introduction to what follows. Through this chapter w can later retrieve this early information and apply it to things we don't understand.