In practice I found retros very useful in my last Scrum team because we iterated on our process to make retros useful. #1 was clustering similar sad faces. #2

In practice I found retros very useful in my last Scrum team because we iterated on our process to make retros useful. #1 was clustering similar sad faces. #2 was as a team, agreeing on actions we could take on those clusters. #3 was tracking how well we did on performing those actions. If it's just venting, it's not overly useful.