Why does "Sonny's Blues" begin with in medias res?

As Kip Wheeler Ph.D. says, "Usually href="http://web.cn.edu/kwheeler/lit_terms_i.html">in medias
res is a technique used to heighten dramatic tension
or to create a sense of mystery." The technique of in medias res,
Latin for in the middle, allows the narrator, whether third or
first person, to introduce background events in strategic positions through
flashbacks for dramatic or
mysterious effect. An example is when the narrator in "Sonny's Blues" flashes
back to the boys youthful anger at the inferiority assigned to them by social and
cultural constraints and to the escape--the forgetfulness--from their anger that they
found in movies. In addition, in medias res allows for a condensed,
compressed recounting of extensive background material so
that a dramatic kernel of a life-long story can be told with a heightened impact as a
short story that has a keen, undiluted impact and focuses on an individual important
theme.