Why use a "load switch" and not just one transistor as a switch
You could use a single FET, but there are several advantages to using a load switch IC.
- Voltages higher than the micro voltage can be switched. (That can also be done by using 2 transistors. )
- The load switch has inrush current limiting built in. This can be done with discrete components as well, but requires more engineering.
- More often than not, load switches have monitoring, such as power good or overcurrent outputs, etc.
- Tolerance analysis is easier when that entire circuit is on one die with guaranteed data on its performance.
As with all things engineering, trade-offs.
In addition to what other respondents have already written, a switch made with a single power MOSFET will have a body diode between source and drain. As a result, the switch can block current only in one direction. In the other direction, the body diode will conduct whether the switch is open or not.
An integrated load switch typically can block current in both directions. This is done either by controlling the bias of the bulk in the MOSFET, or by using two MOSFETs back-to-back.
In this case, the second transistor is performing a level-shifting function. The P-channel MOSFET requires an active-low control signal that is referenced to its source terminal (i.e., across the resistor). The N-channel device allows you to control the switch using a ground-referenced active-high logic signal, which is much more convenient in most applications.