Cooler engine temperatures can have several advantages:

• The intake charge picks up heat from the plenums, throttle body, (supercharger if equipped) and even heads. Keeping these parts cooler keeps the intake charge cooler and thus denser. Density=power.
• A cooler intake charge mixture resists knock better, thus allowing the engine to run more compression/timing/boost. These are usually design factors though.

Cooler engine temps can also have disadvantages:

• The combustion gasses lose heat and energy to the combustion chamber surfaces. Too cool, and you can rob the engine of some efficiency and power.
• As was said, the fuel and timing maps are based upon sensor data. Run the engine too cool without tuning changes to match, and you might be out of the ideal range for performance or efficiency. Same with too hot.
• That was great info from ImportLabSRT about the need to evaporate water out of the engine oil. Viscosity is important too. Factors such as oil weight, bearing clearances, oil pump & piston squirter design, are all temperature based.