The threshold voltage of internal supervisory section of micro mostly is much lower than that of an external supervisory chip. Also, the precision isn't as high. I do always use an external supervisory chip, like MAX1232 for instance, even with the simplest applications, just because I can filter the supply voltage of external supervisory chip if needed. This can be very helpful in noisy systems. Noisy systems can be found, when the micro sees heavy capacitive loads at his ports, for instance, and such a noisy system need not to be large... Then, I put a low pass filter of 4R7 and 100nF...10µF/X5R into the supply of MAX1232 and am shielded against mistriggering.

Why is a precise threshold voltage of brownout detection so important?
Built-in supervisory section with low and unprecise threshold voltage does only protect the flash section from running ill, and by this preventing the flash from loosing parts of the code.
But your application can be in danger in much less relevant undervoltage situations, because the timing specifications can relevantly depend on actual supply voltage. So, in critical applications the system can run ill due to timing errors long before the internal supervisor would notice the mess...

Kai