Why does an author use a subplot in a novel?

to resolve the problem that the main characters face

to introduce the conflict that drives the central plot
to prepare readers for sequels to the original text
to develop secondary characters and story lines

the main character of "the secret life of walter mitty" retreats into a false pretense as a means escape from his mundane reality -- this is a flagrant issue seen in societies abroad, typically found in young adults and it is robbing our youths of their childhood.

explanation:

a thesis -- a well developed, formal thesis is required to be one sentence long. there are many themes in "the secret life of walter mitty" and i had resolved to give you one of the simpler ones while still honing it with class.

the mending wall was written by robert frost

add me on snap   tschrake20

i think the first one, it makes the most sense.