At least half the CAD I do are for just such uses as you have posted. I'll model an item, or a space based on real measurements and then I can sit at the desk and eyeball things for a solution. Even with the vacuum pump, I modeled the little pump, the gauge, the switch to see how I could arrange them in the box.
Next I'll be modeling some racks I need to weld up to hold long metal stock in the shop. Beats my former method of just starting to cut and weld on the fly
Back when I was serious about vintage motorcycle racing I was considering reproducing a four leading shoe drum brake and modeled up this example. Lot of hours invested at my limited skill level. It "functions" too, all the pivots and such. Unfortunately all lost as it was done in AutoDesk Inventor which I no longer have access to.
