When to do a card sort?
Research Insights can tell you what to research next.
I was recently working on a client product. Where we found the 6 out of 10 participants and 2 out of 4 participants from 2 different usability studies were having trouble finding the link to get their task started. They would click around on all kinds of links hoping it would get them to their desired place so they can quickly finish their intended task.
Green Field Products: Another use case for card sorts is more obvious.
When your designing a new product from scratch that does not exist yet. It will be helpful to understand how users group and label categories of information so you know how to organize the information in the navigation and in the design itself.
Examples of Research Insights
Online Tool
I recommend using Optimal Sort. https://www.optimalworkshop.com/optimalsort