he is being called to testify on the Benghazi incident (debacle)
any lawyer knows, the first thing you want to do if someone is going to be giving potentially harmful testimony against you is to discredit them in any way you can
the testimony is not as credible if the person giving it is an alcoholic, crack head, criminal, pedophile, whore, tax cheat, baby eater, etc...
Petraeus is a highly decorated 4 star general his testimony has the potential to be quite damning if it is not favorable to the administration
so they got what they could, he is an adulterer, he cheated on his wife of 38 years and shamefully resigned from his position as the head of the CIA... now his testimony is not as valuable... he is damaged goods