How: Using /bin/flash.lua in openos (or just by using component.proxy(component.list("eeprom")()).set(<code>)), you can flash a program (4096 bytes max) onto an EEPROM. When the computer starts up, this is the code that will be executed initially.
Why: I believe the benefit of microcontrollers and drones is more apparent when playing survival, especially due to cost. Also, by avoiding putting an HDD in a robot, you can use the extra complexity point(s) to put in another upgrade.