Hydride ions, H-, are necessary to oxidise or reduce compounds. BH4- is a very good reducing agent, reducing almost any compound it comes into contact with – but is far too strong for biological applications (NADH/NADPH is normally used). NAD(P)H is able to be targeted to the specific site by cells, and is involved in many metabolic reactions.
NAD(P)H is a less strong reductant when compared to BH4-, but this is better in biological settings as it could cause severe damage to the cell.