Flags are useful in Jolt Lists both for tracking specific events that happen in a list and for setting up the advanced autocomplete feature which can automatically complete certain List items based on the flags that are triggered in a List.
This allows for some advanced automation. For example, you might have an inspection list where you want the person completing the inspection to automatically stop the inspection if there are 3 critical violations. Flags can be setup to track the different violations and then Autocomplete can be used to trigger a List item to automatically mark the inspection as failed when 3 critical violation flags are triggered. You can even have a corrective action that guides the inspector through the process of closing out the inspection early.