[Dave]

It serves as a dust cap. It doesn't affect the sound quality of the speaker if it is removed (unless the voice coil gets crud inside of it). Many early speakers didn't even have dust caps on them. So what do you do if dust gets inside the voice coil? Simple, just turn your speaker face down on a flat surface and play the radio for 12 - 24 hours at a moderate volume level and most of the dust should vibrate out.
Dave