Hey, looks like you’re unsure about what casting is, and what it’s used for.
When I first learned about blueprints and casting, the easiest way for me to understand how it worked was like this:
For example, you have a TriggerBox with an event ‘OnActorBeginOverlap’. When you overlap this box, the box doesn’t know what you are, except that you’re an actor. But you know that the only actor that will cross this box is you, a first person character. So, you can create a ‘Cast to First Person Character’ using the ‘Other Actor’ from the ‘OnActorBeginOverlap’ event.
After casting, you’re able to communicate with your first person character blueprint from the triggerbox blueprint now that you have a reference to the instance of yourself.
For your situation, using Inventory Manager and Item BP’s, you would typically have a situation like this. You have an object in front of you, and you want to know if its an item that you can pick up. So lets say you’re walking along, and you step on this unknown object. At first, your character won’t know what it is, so you need to make a test to see if it’s something you know, like a BP_Item. This is where casting comes in.
On the event that you walk onto this item, you cast that item to a BP_Item. If it succeeds, great, you now know that its a BP_Item and you have a reference to it, so you can interact with it how you want.
However, if the cast fails, that means the item is NOT a BP_Item, and you will not have a reference to it.