Replacement and obsolete parts
The part the manufacturer no longer sells, or wants too much for. Measured, modelled, made.
A machine is down because a $2 piece of plastic broke and nobody sells it any more.
I measure the original, model it, and make it. Where the original design is what failed, I will say so and change it rather than faithfully reproducing a part that was never good enough.
Best case for this
You have several of the same machine, so one modelling job covers a part you will need again.
What I will not do
Reproduce a part that is protected by a design patent, or make anything that carries another manufacturer's branding.