Complex sentence: However, even without actual additions, the fact that a summation is involved "is" of importance: by contrast the empty product, a product of no factors at all, has as value not zero, but one, the neutral element for multiplication.
Split into simpler sentences: However, even without actual additions, the fact that a summation is involved "is" of importance. By contrast, the empty product--a product of no factors at all--has as value of one, the neutral element for multiplication.