Computer-controlled (CNC) machines have progressively joined, and in some cases replaced, manual work across many stages of metal fabrication, bringing a level of precision and repeatability hard to achieve by hand.
What changes against manual work
A CNC machine performs a cut or an operation following a digital program exactly, reproducing an identical result on every piece. Manual work, however skilled the operator, introduces a natural variability that on repeated production becomes a factor in both cost and quality control.
The upfront programming time
The advantage in precision and repeatability has a counterweight: programming the machine for a new part takes time, a cost amortised across higher production volumes. For a one-off piece, manual work can still be cheaper than the time needed to program a machine for a single example.
The effect on the quote
Distinguishing on the quote between CNC work (faster on repeated volumes, with an upfront programming cost) and manual work (more flexible on one-offs, with no setup cost) lets you choose the cheaper method for each job knowingly, rather than applying the same approach regardless of volume.