Why could a UPS device need of a microprocessor?
There are many advantages.  First, an MCU can replace quite a bit of discrete logic and monitoring circuitry for less than the cost of discrete parts or an ASIC.  UPS units need to monitor several different parameters (line voltage, battery charge, load current, etc).  Many MCUs have built-in ADCs, so using a microcontroller means that most of the necessary logic can be implemented in software on the MCU instead of discrete components.  This logic would include things like undervoltage and overvoltage monitoring, battery level monitoring, hysteresis, timeouts, etc.  In general, the MCU will replace a number of discrete logic chips and simplify the control circuitry.  It also becomes trivial to add external status reporting over a serial port or even over Ethernet.  Using an MCU also allows the UPS to be remotely managed - thresholds and timeouts adjusted, output enabled/disabled, etc.

Microcontrollers capable of this would be rather simple.  The atmega328p used in the popular Arduino boards is pretty representative of what you might find in a UPS - 8 bit processor, a few KB of SRAM, a few KB of flash, clock of a few MHz, ADC of a few ksa/sec.  For one that supports Ethernet, it would probably use an ARM-based MCU of some sort, or at least something 32-bit, to deal with the network interface.  Possibly even two MCUs, one for interfacing and one for control and monitoring.  

Personally, I would be more surprised to see a UPS that didn't use a microcontroller of some sort.