What is the use for a microcontroller in a temperature sensor?
While I am not personally familiar with the specific product mentioned in the question, the description at that link clarifies at least some of what the microcontroller brings to the table:


  
  Conversion of a linear resistance change to a standard analogue current / voltage 
  signal from e.g. valves or linear movements with attached potentiometer.
  Suitable in applications with potentiometers that are not fully utilised as the 0 and 100% adjustments on the front can be adjusted individually without interacting.
  


In brief: A basic temperature sensor will demonstrate:


Not necessarily linear resistance change against temperature (or voltage change against temperature, for thermocouples)
For negative / positive temperature coefficient resistive sensors, a resistance relationship to temperature, which then needs some work to convert the output to a linear current relationship, or a linear voltage relationship, with the temperature
Some interaction between set-point adjustments for minimum and maximum scale values if analog methods are used instead of programmable ones, i.e. changing the upper set-point will often affect the lower set point, etc. 


What the microcontroller enables is an output that is linearized (could be a lookup table, some parametric algorithm, or use-your-imagination) and either a current value, used in industrial current loop analog communications, or a voltage value, used in many different systems. Again, from the link:


  Analogue standard current / voltage output of 0/4...20 mA / 0/2...10 VDC. The output signal is proportional and linear to the value of the temperature or resistance value that influences the input.


The "Analogue standard" is with reference to industrial sensing standards.

Of course, both the linearization, and the conversion from a resistance representation of temperature to a current or voltage representation, could be done by an external device, either microcontroller based or analog. Sometimes such massaging of values is even done independently on a computer: Use of a spreadsheet for such purposes is not unknown!

However, for someone whose prime objective is to focus all available engineering time on an end product where the temperature sensing is merely an enabler, a component, devices like the one described are not overkill: How many work-hours of quality engineer time would balance out the premium paid for an industrial temperature sensing device with the flexibility described?