This is my understanding:
Classes hard referenced in a blueprint graph (like using the drop down in the “Spawn Actor of Class” node) get loaded into memory when the blueprint gets loaded. So in Shohei’s example, you would be loading all three classes into memory before you even spawned anything.
The “Load Asset” node lets you load just the one class you need into memory, before you spawn it with “Spawn Actor of Class”.
There is a good explanation of this here.