Why does Monsieur Defarge show Dr. Manette to selected visitors in A Tale of Two Cities?

Monsieur Defarge shows Dr. Manette to the "Jacques" to inspire them to revolution.

As a former prisoner of the nobility, Dr. Manette is a ruined man, a pathetic figure.  Eighteen years of captivity have left him a shadow of a man, thin, spectral, and unsound of mind.  Dr. Manette in his present state is a stark example of the ruthless cruelty and injustice of the ruling class.

Dr. Manette in his debilitated state is such that Mr. Lorry is incredulous when he sees him.  In utter disbelief, he asks Monsieur Defarge, "Is it possible?", to which Monsieur Defarge responds bitterly,

"Is it possible?...Yes. And a beautiful world we live in, when it is possible, and when many other such things are possible, and not only possible, but done...done, see you...under that sky there, every day".

When Mr. Lorry realizes that Monsieur Defarge "make(s) a show of Monsieur Manette", he is at first angered by the thought that Defarge is exploiting the old doctor.  Monsieur Defarge explains that he only shows him to "a chosen few...real men, of my name...to whom the sight is likely to do good".  In solidarity with the common people, Monsieur Defarge and his fellow revolutionaries call themselves simply "Jacques".  Defarge believes that the pitiable sight of Dr. Manette, reduced to this state by the barbarous aristocracy, will remind them of their mission and galvanize them to action (Book I, Chapter 5).