Micropower Voltage Detector, Active Low, Push Pull MCP112

Hi all
I paste the description of this IC:
The MCP111/112 Series are CMOS voltage detectors are well suited for portable, consumer electrics applications due to the extremely low 1uA operating current and small surface-mount packaging. These devices are designed to hold the microcontroller in reset until the supply voltage reaches a predetermined operating level. These devices also protect against brownout conditions that occur as a result of the supply voltage dropping below a tolerable level.
I thought this function is integrated into microcontrollers. Am I wrong?
What does it mean holding the uC in reset?
Thanks

There are many versions, called such as Watchdog reset or Microprocessor supervisor etc.

They are typically used as a hardware safety device, to guarantee the CPU or MCU has a proper reset signal and only starts when the power is stable, or is reset if the program stops doing what it should (a watchdog function).

Many newer MCUs do have capabilities like that built in, but it's not generally in stand-alone CPUs intended for built-up systems.

This is one of the originals, that we used to use & is still in production:

And a memory controller often used with that, in built-up systems - rather than resetting something, it disables the chip select signal and switches the memory to battery power, when the system power fails:

There are others that include device decode etc.