It's a Siege Breaker ability. When you have the pirate flag out, you can spawn special enemies that will cause the skull coins to drop. When you pick up a skull coin, the number above the flag will go up. While the flag number is greater than zero, enemies will respawn and decrease the number. It is a way to kill more enemies faster. Since this is a special level with unique respawn mechanics for the Boops, the flag has been disabled so that the counter never goes up when I collect the coins.
Why kill the crystals on the boop level? Crystal stats are based off of the enemy mob that you killed to spawn it, which affect the experience and money drops. It doesn't really matter anymore since I've outgrown what world 1 can give for experience and money. I don't use my guaranteed crystal spawns on the world 1 crystals anymore, since I have a lot of world 6 crystal cards that I need to collect. I still do this as part of my morning routine (minus the crystals), since the poop boss is progress towards the weekly guild quest of 20 boss kills, and mini-bosses also have deathnote entries now which I think was unlocked from the world 6 sneaking skill. I also like to collect the boop drop, even though it seems I don't really have any use for it. It is something that can't be 3-d printed since you can't AFK the boops for a sample.
