Typical use - supply voltages close to input voltage e.g. 3V from a 3.3V rail. Also as a follow up, i.e. second stage, after a switcher. For example, provide 1.5V or 1.3V from a 1.8V generated by a switcher.

The linear regulator is a type of buck regulator. Vout can not be larger than Vin. The linear regulator functions like a variable resistor. This impedance is varied as the Iout load or the Vin input varies. The current through this resistor is same as the load, Iout. Additional power is required for sense and control circuits. For large Vin-Vout difference, the linear regulator has low efficiency.

A linear regulator can be represented as a variable resistor between the input and output. It can only provide a buck (or step down) function. It can not be used for boosting the output voltage above the input voltage. It requires the least external components, produces the least noise and costs the least due to its simplicity. However, it has the worst efficiency when the Vin to Vout difference is big.