Apart from the start-up instructions, what other programs are stored in the ROM?
ROM is Read Only Memory. It is used to store data/instructions that need not/can not/should not be ever modified (or rarely modified, you can update your BIOS because it is modifiable, but it is rarely done).
Today it is often cheaper to have a CPU and a program to do all sorts of complex tasks than to create custom circuitry (slower, sure; but if that matters depends on exactly what is done). Thus ROM. And that applies to all sorts of things, including some devices; but as devices get more complex, changing said programming is often needed, thus all sort of "firmware" (loaded on startup into the device, not changed until next time).