You would be surprised how much industrial designs that is alive 10 or 20 years after the design. But during the years, a lot may have happened with the software which means that multiple generations of developers may have been involved.

A lamp timer may be a once-off project. Create it. Send all documentation to the factory. Move on with your life.

A large control system may not be a 100k volume project. The single system - or the 10 or 100 installations - may live for a long time, and regularly get new customizations as some peripherial hardware changes. Some additions are made to protocols to teach the system new tricks. Some changed hardware required changed timings, inverted signals, or sometimes really big software updates caused by concurrency issues etc.

The goal really has to be to write the code so that someone who haven't been involved in the project can jump in and figure out what is happening. That level of quality/documentation/... will to a big part be a gain to the original developer. But it will not just be a gain but maybe vital for allowing someone else to be able to modify the code without introducing hard-to-catch errors.

Of course I have seen a lot of code that is both beautiful and easy to understand. And I have seen a lot of code that is just plain ugly, stupid, hard-to-maintain, ... If I like the code isn't controlled by who wrote it, but the skill level of the writer.