With all of his physical and mental deficiencies, why would Grandfather Smallweed make the trip to George's shooting gallery?

Dickens wants to emphasize that the matter of Captain Hawdon's handwriting is of ultimate importance to the plot of the story, and Grandfather Smallweed's painfully difficult trip to see George illustrates this clearly. It also lends some comedy to this chapter, since Grandfather is a unique character who tend to need a good jostling to remember who and where he is and why.