Someone smarter than me can make some kind of rampant crime joke here
I suppose you might think of this list this way: if someone else has them on the battlefield, are you likely to want to get rid of them
C Common Overload Fusion X SPELL Fusion Summon 1 DARK Machine Fusion Monster from your Extra Deck, by banishing Fusion Materials mentioned on it from your field or GY
In the case of Avatar, the 15 most expensive cards would cost an eye-watering $4,230 to purchase, and that's just one copy each and without considering the most expensive card of the bunch, which goes for a staggering $5,300 on its own
Benefits for Your Collection
If you have 4 Deoxys on the Bench, Lugia will still be 20 damage short of the 1HKO on most EX s, so you need to have as much previous damage on the field as possible