The reset flag is used at the start of the import process to copy to the Imported? line item. At that point all list items are showing as false, so when the file is imported, the TRUE flag is populated.
You need to have the Reset to ensure any items imported previously, but now not in the file are cleared
You can also achieve the same thing with the help of a delete action and FINDITEM function.
You want the item that is deleted from the source list to be deleted from the target list as well, this can be achieved by using the following.
1. Create a Module , add the target list as a dimension.
2. Create a boolean formatted Line Item in the same module called 'delete'.
3. Use the formula for the boolean as NOT FINDITEM(<Source List Name>, NAME(ITEM(<Target List Name>))).
The above boolean will be true for all the List items which are there in the target list but they are not in the source list.
4. Create a delete action using the boolean you just created and add the action in a process before you run your import . The first action in the process should be your delete action and the second is your import.