Glad to hear this is working, thanks a lot for confirming!
The correct link should be: Key concepts | Docs, sorry for the 404. The main difference is indeed that the Function Item node loops through all incoming items (whereas the Function node gets executed only once).
In the above example workflow this would allow you to perform the lookup for multiple items as it would check for each input item whether the value appears in the referenced array.
Hope this makes sense 