there are many real world scenarios for any size of buck converters, for beginners to mention is the higher efficiency; 5V from 12V with a LM7805 and 1A current draw results in a loss 7W while only using 5W - an efficiency of only about 29% ; with an efficient buck converter you can reach 95%
not only you save energy, you save on heatsink size as well.
somethin what you cannot do with linear regulators at all is upconversion, so from lower to higher voltage, a boost converter, or from a positive voltage to a negative voltage - an inverter