In the example you’ve got the SpawnActor thing already outputs a BP_FloorTile reference, you can see that by the GetAttachTransform node says “Target is BP Floor Tile”. This should give a warning when compiling with the cast node saying something like you don’t need to cast.
But in general when casting you go from something general to something more specific, so if you create an actor MySpecialActor with a variable in it, only that type of actor will have that variable, so you will need to cast to it in order to access its variables and functions.