When designing low power devices we want our design to be efficient and consume as little current as possible. Sometimes you do not find power efficient sensors that you can sleep or put in low power mode, in these cases is when we need to use load switches to turn off the sensor completely off. But you need to be careful in choosing such devices because they will consume some power while they are OFF.
Please take a look at this application report from Texas Instruments, It has some good information and guidance of Load switches.
And please take a look at the Load Switch section of Texas Instruments.