Question - Why use a signal converter for Pt100?
Answer -
The temperature sensor pt100 operates on the resistance change; i.e. if the ambient temperature is0oC, the equivalent resistance of the sensor is 100ohm.
When the thermostat pt100 is connected directly to the PLC, it must be connected to the power supply for operation; make long wire.
And also 4-20mA is the internationally accepted standard.