Their working domain. If you're a developer in financial services, learn finance. If you're a developer in oil exploration, learn seismology. Don't constrict yourself to only being a programmer who writes whatever code you're told to. Be a holistic problem solver.
It's easier to write code when you understand the context. The customer requirements aren't always flawless and might make assumptions about things that you don't know.
During my career I've had to study:
- electrical engineering (phase angles, current transformers etc)
- finance (payroll and accounting)
- building management (maintenance schedules etc)
- mining (terminology, how explosives are set in underground and open mines)
I'm not an expert on any of those, but I know enough to be productive and know _why_ I'm doing the task I'm assigned.
Even if not an architect, it will make you a very senior and respected developer among the other smart people that can actually evaluate this. And if in your current company there are no such smart people, it will open the doors for you to find another company where those skills will be rewarded.
Understanding your users, and what you have to solve for, better than they can probably explain it to you. This makes both your and their lives easier.
To have more experiences and expand your horizon. You never know where it will take you next. Even if it does not translate to more money, it will definitely give you a sense of self-accomplishment and MAYBE make you wiser.
