biochemistry - NADH vs. NADPH: Where is each one used and why that instead of the other?

The phosphate group in NADPH doesn't affect the redox abilities of the molecule, it is too far away from the part of the molecule involved in the electron transfer. What the phosphate group does is to allow enzymes to discriminate between NADH and NADPH, which allows the cell to regulate both independently.

The ratio of NAD⁺ to NADH inside the cell is high, while the ratio of NADP⁺ to NADPH is kept low. The role of NADPH is mostly anabolic reactions, where NADPH is needed as a reducing agent, the role of NADH is mostly in catabolic reactions, where NAD⁺ is needed as a oxidizing agent.

You'll find some more information about this in chapter 2 of "Molecular Biology of the Cell by Alberts et al.