Using reset controllers with modern microcontrollers
You only mention brown-out as a reset condition. However, in some systems, there could be multiple reasons for wanting to reset the processor. In which case, having a separate chip to monitor all those reasons and either reset the processor or provide it with some type of notification could be of benefit.

For an extreme case, take a look at the Lattice ispPAC-POWER607. It is capable of monitoring six power supplies, digital I/O for manual reset, and external watchdog circuitry. It also has it's own internal timers, the power supply monitors are completely programmable, it has an embedded PLD for logic code, and it has FET drivers to control the power supplies.



Like I said, that is an extreme example. Especially if all you need is brown-out detection. But if your system has a more complicated reset scheme, then having an external programmable controller can be very useful.

And there are, of course, chips to suit any need in between the three pin single voltage monitor and the 32-pin fully programmable system management chip.