'Rubber-like left hand'

Here, Harper Lee is establishing that Tom Robinson has a debilitated left hand, because this information becomes critical at a later point in the trial.  By asserting the state of Tom’s left hand early, when his crippled hand becomes pertinent to the trial, (When it is deduced that a left handed person had to have beaten Mayella) the reader is able to use their knowledge to come to the immediate realization that it can’t have been Tom who beat Mayella.