As a QCAD user I wondered what I was missing by not spending five thousand quid on AutoCAD 2019!
The answer is AutoCAD supports a wider range of drawing types plus many time-saving features useful to professionals. For instance there are huge libraries of industry standard parts the draughtsman can drop into his drawings rather than create himself. Also 'workbenches' tuned to particular drawing needs: Architectural, Plant, Electrical, Maps, Mechanical, and a combined 'Mechanical, Electrical, Plumbing' workbench. Each workbench is full featured, including automated drawing processes. A busy drawing office would benefit. An Office Block would be designed far faster in Autocad than, say, QCAD, not because QCAD doesn't have the primitives needed to produce the same drawings manually, but because AutoCAD is thoroughly 'power assisted'.
Apart from the price of AutoCAD, the other downside of high-end packages is the extended learning curve. In contrast, QCAD is moderately well supported with libraries, and although it has all the basic drawing tools needed to produce 2D engineering drawings, it doesn't have complicated extras. I'm happy with QCAD in the sense I don't need GPS, Radar, Air Traffic Control and a Flight Computer to get to my local supermarket! Quite likely a Model Engineer would never use AutoCAD's Plant, Electrical, Architectural, MEP or Mapping capabilities. Most of us I think would be content with a straightforward 2D tool.
Dave