It won't affect performance at all. You can spawn/destroy hundreds of objects in a single tick with no performance issues.
Since you have to have an object in order to spawn an object, that object has to exist somewhere. Sticking it outside the level is just a way of getting out of your work space. You can stick it on a different layer and hide/lock the layer if you want.
Or you can put that first monster in your game level where you want it to appear, and just don't destroy it. Then use spawning objects, or an array, or an .ini, or whatever to create the rest of your monsters. It's entirely up to you, that tutorial just shows one way to do it.
Or if you really want to you can just copy/paste your monsters all over the place in the layout wherever you want them. If they have more than one piece, though, it's easier to use spawn points, so you're not copy/pasting several objects and placing them all just right.