Nothing, really. You simply don't have to swap after every shot.

what prevents me from putting homing -> 10x magic missiles and switch wands between shots?
100 uses of homing magic missles.

am I missing somethig here?

Correct, but that's not something you really want to do in a heated situation.
This spell should also work with limited modifiers like Matter Eater as well.

You can swap away and back to the wand and it will automatically reset the wand and when the modifier is drawn, it will apply the modifier to the first available spell in the chain, skipping empty spells. This is nice if you don't feel like swapping every time, though.

Oh nice, could honestly be integrated into vanilla!

For example, let's say you want to put homing on a magic missile. You don't have Unlimited Spells, but you do have like eight copies of magic missile. Plenty.
If you put homing and then all of your magic missile copies, only the first one gets homing applied to it, and the rest don't. Unless you multicast all of them, but then you burn through your charges in no time. If you only put in one copy at a time it will always have homing, but then you have to run back to a Holy Mountain to swap it out every time it runs out of charges. Completely unreasonable.

With this, you can just slap all the copies in there. You'll cast one with homing and the rest are skipped, so you get to use them one charge at a time and don't have to worry about "reloading".

Wait, what advantage does this offer over rearranging the spells based on uses?
...Or just not using it at all and knowing the game will skip over depleted spells anyway?

Not at my fullest mental capacity right now so I'm sure there's a use case I'm not realizing

I love this, thank you for your service
no more fiddling with black holes