Time & Money.  It all comes down to time and money.  Programming complex parts by hands is becoming less common due the development of post-processing software in CAM packages.  Manufactures need to be cost competitive or run out of business.  Parts can be modeled in CAD, tool paths defined in a CAM package, and g-code cranked out by the post-processor.  ChatGPT can now look at a mechanical drawing and crank out g-code.  How do you know if the post processor was written correctly for a new machine?  Knowing g-code lets you debug programs when parts aren't machined to spec or a post-processor for an older machine creates code that won't run. 

For simple changes or simple parts, it might be faster to tweak an existing program. 

When seeking to diagnose why a part is not to spec, a machinist needs to check the setup, tools, and part program.

• X - Optional X-Axis motion command

• Y - Optional Y-Axis motion command

• Z - Optional Z-Axis motion command

Machine will move to the specified position rapidly.  Only for non-cutting use.

For use when cutting, machine will move to the specified X, Y, Z position at the specified feed rate.  This command is modal, i.e. all subsequent moves will utilize the same feed rate unless it is changed.

G01 X1. Y2.5 F20.

to home Z-axis::

G91 G28 Z0. (GOOD)

- or -

G53 G00 G90 Z0. (BETTER)

 never G90 G28 G00 Z0. --> may crash machine

To Home X,Y Axes

G91 G28 X0. Y0.

Requires a subprogram

see https://www.haascnc.com/service/codes-settings.type=gcode.machine=mill.value=G150.html

Links to an external site.