Because triangular prism ABC-A'B'C, quadrilateral BCC'B' and ABB'A' a' are parallelograms.

As shown in the figure: VA-BCC' = VA-BCB', VA-BCB' = VA-A' B' C'

So: va-bcc' = va-BCB' = va-a' b' c'

Triangular prism ABC-A'B'C' volume = 3 * va-bcc' = 3 *1/3 * s △ bcc' * a =1/2 * s quadrilateral bcc 'b 'a

Simply put: V=a*S/2.