I am wondering what is the purpose of an IC such as the MC34064. Is it just to ensure a reset of microcontrollers upon power up or are there other reasons? I used to make a simple transistor switching circuit (with RC) for resetting microcontrollers in the old days. I am asking because the name of this IC is "undervoltage sensing" and not "reset IC" or something similar.
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.
In a normal usage, the user will press the power button to turn a device off. When the MCU sense the button press, it will initiate shut down sequence and the firmware should set the pointer to the beginning of the program.
When there's a power outages or maybe the user just switch off the plug. The undervoltage pin should able to sense these disturbance before the MCU run out of power from it's bulk cap. The firmware should quickly set the pointer to the beginning of the program as to prepare for the next power up. So when next time the device is power up, it should work as normal.
If you're using for simple IO commands, you may not need it as it's ok to continue running from where the pointer last location. However if you need to initialize a lot of ICs using I2C for example, then initialize pointer to the beginning is important.
Undervoltage situations can be bad for microcontrollers and memories. Many microcontrollers have a basic undervoltage protection system built in for this reason. The most common failure mode is corrupted flash memory, due to low voltage making the flash memory write controller malfunction.
Another common use for undervoltage sensors is to protect batteries. Many types of battery do not like being discharged to very low voltages, either because they leak or because they cannot be fully recharged afterwards. LiPo and NiMH cells are damaged by over-discharge, for example. It can be hard to get a complex circuit to go into a low current state when voltage gets low, but an undervoltage sensor connected to a FET between the circuit and the battery is quite effective and cheap.