'In The Miller's Tale... Chaucer is concerned primarily with the
telling of a farcical story.'

Do you agree? Discuss Chaucer's purpose in this tale.

Sounds like a homework question, which isn't supposed to get an
"answer."  Here's a few hints:

1. If the question/statement were "true," they probably wouldn't have
asked.  It (the story) also probably wouldn't have been included in
the book.

2. Read the between-story interaction just before the Miller's story
carefully.  If you are reading a decent educational text containing
Chaucer, hopefully it has a few side/foot notes about word use that
will help.

3. Compare the Miller's story to the Knight's story to see how he
carried out whatever purpose he may have had.

4. Compare the characters of the knight and the miller to see what
purpose Chaucer may have had for including the Miller's story and
character.