Sly: What's with taking out the disco ball?
Bentley: Its impact will shake the Nightclub's front peacock sign loose from its moorings... look I can't talk now. I've got to keep moving, keep safe!
― Why the disco ball must be destroyed[src]
Bentley needed to enter the discotheque to destroy the disco ball.
Bentley needed to drop the disco ball in order to loosen up the nightclub's peacock sign. To do that, he bombed all four pillars that supported the ball. With all pillars destroyed, the ball crashed on the ground, and the peacock sign became successfully loosed from its moorings as expected.
Behind the scenes
This is one of the jobs that appear in the Sly 2 demo included with the PlayStation 2 release of Ratchet & Clank: Up Your Arsenal.