During the manufacturing process, a chemical called Mold Release is applied to the parts of the glass lense that they don't want the silicone to adhere to. On release from the mould, the excess silicone is trimmed and peeled off, but the mould release (mostly a wax) is still on the glass. This wax makes the mask fog up more readily.
This is where Sea Buff comes in, it is specifically designed to remove this coating and help prepare your new mask for fog-free use.
It is also great for older masks that have begun to grow a grime coating on the glass or for cleaning slates.