You need to cast something (in the object pin) as that Blueprint. You’d generally use a cast like this if you were getting an actor the game was not able to recognise as a specific bp type on its own (like say, if you were using the “other actor” pin from the begin overlap and you wanted to trigger a custom event in the specific actor doing the overlap, you would need to cast it to the right bp for it to be able to get the variables or custom events that are inside of it)
So basically get all (actor of class) or (actor with tag) works if you want all the actor in your level that fit a criteria (have a specific tag or class), but if instead you want to use a specific instance of an actor bp, you need to get it in some way (say, from an overlap) and then cast it to its actual bp type for unreal to understand what it is.