What is purpose of an Undervoltage Sensing Circuit?

I guess you are trying to understand why you need an IC to do what a simple transistor could do "in the old days", which is pulling a reset pin low (or high), right? If that's the case, a couple of reasons are:

• The trigger point can be precisely set, as the IC has its internal bandgap reference voltage. With a simple transistor, it's harder to trigger at a very specific value reliably.

• You can set a specific time for the reset pin to stay low when it's triggered (which is harder to do with simple transistors). Many ICs have strict requirements with minimum reset assertion time.

• This type of ICs usually operates down to low voltages (in this case 1V) guaranteeing that it will work reliably for large voltage drops on the power rail.

The fact that they label it as UV sense instead of IC reset doesn't really matter as they clearly describe the purpose that this IC has been designed for, which is to reset a microprocessor.

From the product page at On Semi website.

."The MC33064/MC34064 is an undervoltage sensing circuit specifically designed for use as a reset controller in microprocessor-based systems."

There is a bunch of voltage-sensitive activity inside a MCU, and not all activity has problems at the same (out of spec) VDD.

A precise UVD ensures all the activities get the desired VDD, even tho some are still functioning properly.