A: The number of traps that trigger each round is randomly chosen by the Evony server at the beginning of each battle, so you don't know exactly what it will be for a particular battle. The rate option allows you to try some different trap rates. Setting this value in the calculator is like pretending that the server will actually use that value for the real battle.
The main reason to have it as an option is so you can try different values to see if it effects the results of a battle. In many cases it will not change the results much. But in some cases, espeically when attacking layers are thin and there a lot of traps, a different value could have a large impact on the results. If you do see a large effect when changing the trap rate you should realize that the result of the battle is uncertain and relying on one outcome or another is a risk.
Note that the actual trap rate is typically between 4.0 and 9.0.
A: The kill power of trebs vary from battle to battle. The "pow" pull down is mainly for debugging at this point but you can use it to vary the power of trebs if you find it useful.