If the model you are cutting has fine detail you are going to need to use a small tool to be able to machine that detail - a sculptor would not use a large chisel to sculpt fine detail. However with a small tool there is a limit to the amount of material that can be removed - again using the sculpting analogy a sculptor would use a large chisel to remove the bulk of the material.
The roughing page allows you to specify the machining equivalent of the large chisel - a large tool. This tool will used to remove the majority of the material ready for a final pass by a smaller tool. On this page you can specify the tool to use and the maximum cutting depth for the tool to ensure that too much material is not removed in one go. If you attempted to machine the whole depth of a relatively deep model in one go you may break the tool or exceed the capabilities of the motors on your milling machine.
