At a critical juncture in the rebellion, a dramatic and irreversible act is needed to solidify the rebel son's power and permanently sever the bond with his father. The counsel comes from Ahithophel, regarded as the wisest man in the nation, yet driven entirely by cold, immoral political calculations. His advice is designed to solve a deep crisis of confidence within the rebel camp. The primary approach among commentators is that Absalom's supporters are paralyzed by fear and losing their resolve. They worry about the natural possibility of a reconciliation between the father and son. If peace is made, the rebels will be left entirely exposed to the vengeance of the experienced King David [רש״י, מצודת דוד, ביאור שטיינזלץ].
This fear of a truce is amplified by Absalom's prior reputation as a God-fearing man and a Nazirite, which makes a peaceful resolution seem quite likely [אברבנאל]. Furthermore, the fact that David left his concubines behind to guard the palace signals that his heart still leans toward his son. By doing so, David expects Absalom to protect his property as a rightful heir, rather than harm it as an enemy [מלבי״ם]. Ahithophel himself is terrified of such a reconciliation. He knows that if David and Absalom make peace, he will be the first to pay the ultimate price for his treasonous advice. Therefore, before proposing his main, highly dangerous plan to kill David, Ahithophel must manufacture an absolute, unbridgeable rift to ensure Absalom cannot back down [מלבי״ם].
The proposed solution is for Absalom to sleep with his father's concubines. Taking the former king's concubines serves as a highly public, symbolic declaration that he has entirely replaced the monarch and seized absolute power [ביאור שטיינזלץ]. Ahithophel advises him to take more than one concubine and to perform the act publicly, guaranteeing that the news will echo throughout the entire nation [אברבנאל].
The ultimate goal of this act is to make Absalom utterly abhorrent and hated by his father, akin to a foul stench [מצודת ציון, ביאור שטיינזלץ]. Defiling a father's bed is a permanent corruption and a severe sin that eliminates any future chance of forgiveness, much like how Reuben lost his birthright for a similar offense [מלבי״ם, אברבנאל]. Once the public realizes that the damage is permanent and the rift is final, the hesitating supporters will understand there is no turning back. They will regain their courage and confidently unite behind Absalom [רש״י, מצודת דוד, ביאור שטיינזלץ].
Beyond Ahithophel's calculated political maneuvering, this event represents the tragic fulfillment of Nathan the Prophet's harsh prophecy to David. The prophet had foretold that David's wives would be taken and violated in broad daylight. Thus, David's own past sins set off a chain of events that directly culminated in his son's terrible act [אברבנאל].