For more than 2 years, after running the simulator in Easel, I download the G code file then go to the Onefinity program, upload the file and run the file on the CNC, works seamlessly every time. What would I gain by using a post processor?.
If you aren’t having problems, then you probably don’t need one. If you are using a different cam program, you need to get the gcode into Grbl format which doesn’t accept all of the codes available. A post processor tweaks the code tailored to a specific machine’s needs. For example if you were to use say CAMBAM, you need to select Grbl as a post processor to get the syntax that Grbl likes as opposed to another one. Fanuc is an old version that most machines can handle. From my experience some software doesn’t like arcs in a XYIJ format, and needs to convert that into small straight lines. Cambam has a selection called “arcs to lines” if I remember correctly.