Whats the purpose of this baffle as showed in crutchfield... and what materials can be used?

 

Looks like its there to cover up the hole where the stock speaker was. Stock speakers are 6x8 if im not mistaken, so putting 6.5 or 5.25 takes up less space, so you need to cover that space up to or else you'll lose sound.

 

If thats all they used to blcok of the 6x8 mounting hole its a shame.

That could be used to help isolate and seal vibrations and sound from the driver or theadapter.