What is it needed for

So, your company has bought a new CNC machine.
And your task is to launch as soon as possible 
him to work and produce products.
But in order for the machine to spin, it is necessary to carry out
technological training, especially
how to get control programs for this 
machine. 
You can make programs manually, but this is the last century.
Now there is a lot of software for CNC machines.
Such as CAM systems (SolidCam, Powermill, FeatureCam, ArtCAm, NX, MasterCam, etc.), 
with which you can completely 
do the processing on the computer, pick up the cutting 
tool, tooling. Check for gouges, residual allowance,
collision .Design the kinematics of your machine tool based on
on the 3d model of the machine. Completely check the machining simulation with
machine tool in the graphics window  CAM systems, get a list of tools 
This makes it possible to avoid 
breakage of the cutting tool, collision with the basic units of the machine. 
And also while the machine is working, fulfilling the production plan, you are on 
computer, you make a processing project for the next product and completely 
you check it. This ensures safety and security 
the correctness of the calculation. 
Now let's move on to the question, why do you need a postprocessor. 
When you have already done a processing project in the CAM system, the final
the first step is to get the control program for your machine.
You choose a postprocessor and use it to get control 
program. In other words, this is a translator file that translates 
processing on a computer into a program that your 
machine. 

Exactly  the postprocessor makes the control program understandable to the CNC system of the machine -  where the working bodies of the machine need to move, along what path, when it is necessary to change the tool, and which cutting modes to activate - spindle speed, feed, cooling on and off, transfer and capture of the workpiece from one spindle to another and much more. All this depends on the CNC system and machine automation.
     When the head of an enterprise decides to purchase new expensive equipment, he must know that, in addition to this equipment, he needs to have the means to obtain control programs specifically for this type of CNC and machine model. Once again I draw your attention - this is a CAM system and a post-processor developed directly for your CNC system and for your machine model, taking into account the kinematics of the working bodies, the configuration of the axes and much more. The quality of the products and the safety of processing will depend on how professionally the postprocessor will be developed. With flaws in the postprocessor, at certain moments, unforeseen situations may arise when the movement may occur in a completely different direction from what was intended. In this case, at best, a defective part can be obtained, and at worst, there can be a serious accident of expensive equipment.

  For the development of control programs, I recommend the well-known CAM system 
Autodesk  FeatureCAM is a user friendly and easy to use CAM system
FeatureCAM allows you to program 2D and 3D milling, 5-axis continuous, high speed, turning, turn/mill, and EDM.
Autodesk Powermill is also very  convenient CAM-system, which is designed for milling parts of complex shape.
Each of these CAM systems requires a post processor. But if you really get it from a professional developer and it is implemented exactly on your hardware, then believe me, you will forget about all the troubles that could happen when using dubious unverified post processors.