1- to identify the best situations for a protocol to be used.
2- identifying advantages and disadvantages of the protocol to improve it and compare it with related protocols.
3- gives a better understanding to how protocol performance may change by changing some attributes.
Finally I think a better evaluation may need the usage of multiple simulation tools, just to be sure that results are similar regardless of what tools are used.
I think because there are some parameters that are difficult to be represented by another phase such as mathematical phase .The simulation can be the best for representing them.