Please see this example below :
This is very simple example, why we need Read List. Explanation :
1. This workflow is scheduled every 15 min. The requirement is that it should only read the data from the database only once when it starts first time. Then it should send the delivery every 15 min to same group of recipients which came in first run of workflow.
So, how to achieve this, because data in the initial query (1. Query) may change everytime you run the workflow.
2. So, to implement this, when we will start the workflow first time, we will create a list of that data in the List Update Activity (2. List Update).
3. Then we will simply read the list again and again after sending the delivery each time by implementing some logic. So, that only the same group of recipients will receive the delivery.
Note # This workflow is just to clear the idea behind Read List to you, it may or may not run.
I hope this clears your doubt.
Another example (that a lot of our customers come up against) is where you have been given an external file from another team or partner - let's say its a csv file that contains a list of email addresses that should be excluded from a campaign. You can upload the csv file into a 'list' and then inside of a workflow use the read list, to make the data available within the workflow.
But the Excel File (.csv) can also be read by "Data Loading" activity, I don't think there is much need to use the Read List activity for this.
That is a different use case, the one I described is valid. For example list's used by multiple workflows.
In these forums, it is good to provide new comers to the platform with examples of real world use cases and let them decide the route. I tend not to make statements like 'dont think there is much need' but thanks for your input.