What is the reason for different temperature thermostat

I think the OP was asking why thermostats are available for his outboard engines where the thermostat is designed to open at different set coolant temperatures.

The quick answer is it depends roughly on the kind of load the engine will see, and the temperature of the water used to cool the outboard engines. You want the engine warm enough to run efficiently, yet not too warm that the lubricating oil gets too hot and starts to lose viscosity.

 

The different values are the temperature at which the thermostat will start opening, therefore sending hot coolant to the radiator. The lower the temperature, the cooler the engine will run.

It is just easier to change the thermostat when going from a hot environment to a cold one (thus, more cooling from the radiator), than reducing the size of the radiator or the flow of the coolant pump. This way the engine can be kept at the same running temperature no matter the conditions.

For example, say you have the 50°C thermostat and the outside temperature to cool your radiator is 20°C. The coolant temperature at the exit of the radiator might be lowered to 30°C (let say it is the engine designer's intention). But if you use the same engine where the outside temperature is 5°C, the coolant temperature will drop to 20°C at the radiator exit, which might be considered too low when re-entering the engine block. But if you put the 71°C thermostat instead, your temperature at the radiator exit will be back to 30°C.