Wireless Channel Modeling and Channel Estimation
You never know the actual wireless channel that your system  will face while working. You can assume a certain statistical model but even if this model is correct you will not know its exact parameters in advance. If you are doing simulations, then you can assume the model and its parameters to be able to complete the test and the parameters can be selected to simulate the real live best and worst scenarios to get how your system will behave in both cases but again this is only a simulation and the real operation will bring different channels with different coefficients.

In real life you want to get rid of channel effect, and to do so you need both channel statistical model and parameters. The estimation step is used to get these parameters as accurate as you can considering the system limitations and those parameters will feed the equalizer which should cancel channel effect and which is build based  on the statistical model of the channel.