Most rotary knobs are resistive potentiometer that have moving mechanical parts that tend to wear out over time and also change the electrical characteristics. This will affect the performance of your analog TIG welder.
Below is an image of the internal of an rotary knob
With rotary knobs it is hard dial in the settings. Due to wear and tear their is additional variations
Alternatively digital controls has minimal mechanical components are designed to last a long a period of time with minimal wear and tear. Below is an image of a digital membrane switch.
These membrane switches will always command the an accurate settings. This is will improve the performance of the welder.
Below is a cross section of a digital membrane switch
Depending on the step sizes it is possible to accurately adjust the settings. It is much harder to achieve high accuracy with analog rotary knobs
Also digital controls switches tend to be cheaper compared to matching rotary components. This is also an advantage. The electronics technology is advancing every day, this transitioning over to digital can provide better products are cheaper price.
Additionally digital TIG welders had additional features to improve the performance. I suggest checking out the below references for details