Why do you think Charles Dickens decided to include Mr. Joe's visit to London?Great Expectations by Charles Dickens

Since Joe Gargery has been a surrogate father to Pip, he
naturally thinks that Pip will be delighted by a visit from home.  However, when Pip
receives a letter written by Biddy, he becomes very
anxious

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...with considerable disturbance, some
mortification, and a keen sense of incongruity. If I could have kept him away by paying
money, I certainly would have paid money to prevent a visit from
him. 

Pip worries that Joe
will disapprove of his spendthrift habits, and he is most concerned that Bentley Drummle
will disapprove, not to mention Herbert Pocket, his roommate. When Joe does arrive, he
is most uncomfortable because he senses that Pip disapproves of his being in London. 
Realizing that Pip is now a gentleman who does not wish to be associated with a
blacksmith, and addressing Pip as "Sir,"Joe tells Pip he will not
return,

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"Diwisions among such must come, and must be met
as they come. If there's been any fault at all to-day, it's mine. You and me is not two
figures to be together in London; nor yet anywheres else but what is private, and
beknown, and understood among friends....You won't find half so much fault in me if,
supposing as you should ever wish to see me, you come and put your head in at the forge
window and see Joe the blacksmith, there, at the old anvil, in the old burnt apron,
sticking to the old
work...."

It is this
admission of social inferiority that makes Pip feel ashamed. Instead of being
subservient to those of the upperclass like Pumblechook, though, Joe displays "a simple
dignity" as he informs Pip that he will not return to London, but, instead, remain at
the forge where Pip can visit him.

Clearly, Pip has become
snobbish because of his wish to be a gentleman, feeling now that he is superior to those
who do not have money and education.  He is concerned with his social position, even
worrying what the boorish Bentley Drummle might think if he should see Joe, who,
ironically, is more of a gentleman in his heart than Pip.  It is only when Joe mentions
that Miss Havisham has asked him to tell Pip that Estella wishes to see Pip, that Pip
shows interest in what Joe says during his visit.  Nevertheless, after Joe has departed,
Pip does try to catch him, but Joe has gone too far.

With
Great Expectations as a satire of what Dickens considered a
frivolous aristocracy, there are several targets of this satire in Chapter
XXVII.