Digital circuits require some minimal voltage in order for their internal transistors to switch. This is because semiconductors only conduct in the presence of an electric field with a high enough potential to excite and free up the electrons hanging out near their junctions. That means that if your voltage drops below, say 1.8V (common for most low-power digital circuits these days), you won't be able to use your device any more.

There are some very efficient buck/boost converters out there. Like 95% and above efficient. Typically the cost of the power to use a buck/boost dc-dc converter is far outweighed by the ability to convert more of the stored charge into a potential that's suitable to operate the circuit.

Put differently, picture a barrel with a tap on it's wall somewhere near the bottom, and a different barrel with a conical-shaped bottom which tapers into a drain-like tap. Which is more useful? The latter is what you get from a good buck/boost converter.