The speaker grilles are completely cosmetic as a long design staple of the PowerBook/MacBook Pro design. First seen on the 2001 PowerBook G4 Titanium, the grilles were smaller as about the actual size of the speakers beneath them. But the aluminum redesign that came out a few years later pretty much cemented the current look today of wholly flanking the left and right sides of the keyboard. Back then, though, the case design had minimal vent holes on the bottom, so the grilles pulled double-duty as extra ventilation intake. Even back then, of course the speakers were never big enough to be the ACTUAL grilles themselves (see the subtle darker circles visible in the grilles in the linked picture. Also the ambient light sensor in the left grille.) And this continued on through the unibody design phase even after large vent intakes were added in the Retina-onward models. By that point it was a recognizable part of the MBP’s design language.
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