Why one battery voltage is never enough
A robot's battery pack delivers a single voltage, but almost nothing else on the robot wants that exact voltage. Motors are often happy running close to pack voltage, since motor drivers are built to handle a range and higher voltage generally means more available torque and speed. But the logic-level electronics, microcontrollers, single-board computers, most digital sensors, typically expect a clean, tightly regulated 5V or 3.3V, voltages far below a 3S pack's 11.1V or a 4S pack's 14.8V. Feeding raw pack voltage into a 3.3V logic pin doesn't just fail to work, it destroys the component almost instantly, so a robot needs a way to convert one voltage into several different, stable rails.
This is the job of voltage regulators, and robots generally use switching regulators (buck and boost converters) rather than older linear regulators, because switching regulators are dramatically more efficient at converting between very different voltages. A linear regulator drops excess voltage as wasted heat, which is fine for a small voltage difference but disastrous for something like an 11.1V-to-3.3V conversion powering a current-hungry compute board, where most of the battery's energy would turn into heat instead of usable power. A buck converter steps voltage down (pack voltage to 5V logic, for example), while a boost converter steps voltage up (useful when a partially depleted battery's voltage sags below what a rail needs and the regulator must raise it back up).