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.