At the end of the day, both CNC machining and conventional machining aim to achieve the same goal using the same raw chunk of plastic or metal. The difference between the two is how they are able to achieve this goal.
Conventional machining can be effective when it comes to small-quantity projects, especially since it can typically cost less than CNC machining. A highly skilled operator sets the gears during the machining process to help create a part that is as close to your requirements as possible.
If you need a large quantity of precision parts and want them quickly, then you want to look into CNC machining. Fully automated, this process improves speed, production rate, and accuracy when compared to the manual nature of conventional machining. This makes it the best choice when you need to create a quantity of parts all at once or need something with complex concave or convex arcs, imperial and metric threads, or two tapers of differing degrees.
The key to deciding between CNC machining and the conventional option is how many pieces you need. The more significant the amount, the more you should consider CNC as your machining option.