What could be the usage of material declaration datasheet for Spartan-6 package?
The materials declaration is used if your product needs to comply with RoHS or other regulatory restrictions on material content. 

RoHS not only requires that your product not contain certain materials, but also that you document this fact. Typically, in order to do that, you need to request the vendors of all the components in your design to provide a material declaration like the one you showed. This documentation isn't just needed for the chips but also the pcb laminates, the soldermask, the solder, the adhesive stickers with the serial number, etc.