UCAs are hired to help administer the course in a better form where students who have previously taken the course and done well can help the professors conduct the labs and discussion sessions and interact with the currentr students to make things easier for them.
I was the UCA for CS 121 - Introduction to Programming in the Fall of 2019 and I will be the UCA for CS 240 - REasoning Under Uncertainity in the Fall of 2020.