Word of the optional install first came from the official Black Ops 2 forums, where a development team member – who we have confirmed was indeed Game Design Director David Vonderhaar, as listed – announced the option.
As explained to Joystiq by Activision representatives, the install option is reserved for the PlayStation 3 and is intended to reduce strain on the PS3's Blu-ray drive during extended gameplay sessions. Unlike the option to install a texture pack in EA's multiplayer shooter Battlefield 3, the Black Ops 2 install will not alter the quality of the upcoming game's graphics.
According to Vonderhaar's forum post, the feature was implemented because of fan demand. "When we explained to PS3 folks that installing to the HDD wouldn't neccessarily [sic] give us any significant performance gains, they explained back that the issue is they play so much it puts constant stress on the Blu-ray drive and wears them out."
Vonderhaar adds that the install is "a non-trivial amount" of HDD space, but wouldn't give an exact number because "anything can, and does change."
Joystiq is still waiting on word about how the install will affect load times along with a few other details; this post will be updated when Activision offers a response to those additional questions.