Test Tools are needed for Troubleshooting and Repairing Electronic Gadgets, Equipment, Electrical Panels and Machines. Precision Instruments are required for calibration of field and machine control electronics. These are also required by the R&D engineer for design and development of new products and trying out new concepts.
In Production testing, Production Engineering, Quality Assurance QA-QC etc. You need to build Test Jigs, Configure Automated Testing depending on the size of project or budget allocated; yet it has to be made flexible, scalable and modular. These require instruments or blocks that can be adapted and reconfigured or even interface with a computing network for automated testing, reporting and analysis.
Special Electronics come with every tool or gadget these days, A Slide calipers has a LCD display, Digital Readouts on mechanical instruments can help measure even sub-microns easily. Analytical and Chemical Instrumentation is another specialized niche. Then we have medical instrumentation which is more demanding in quality and requires sensors that are small, very sensitive and safe.
Instrumentation goes into all these and beyond. The day we find water on mars, It is only because of a precision instrument at the helm of things.
These are basic measuring circuits you can build for your home lab. I have documented a lab DMM i had built for my use many years ago. the circuits may have errors, build it only if you understand.