Thirteen originally didn't plan to use the Capture Spheres to catch the Brain Beetle.
Because if he used it, he wouldn't become their Master or anything.
At most, it would only increase his Apocalypse Points since it counts as him completing his mission from the Order of the Apocalypse.
The stronger the monster, the higher the points that he would get.
It was also a life-saving item that could help Thirteen survive a battle against Rank 8 Monsters and below.
Unfortunately, the Capture Spheres he had weren't strong enough to capture Rank 9 Monsters. It seemed that Metatron knew that Thirteen would abuse them if he could get away with it.
The only saving grace was that he could ask for an item once a year and would be given the chance to get more as long as he managed to open another layer in the treasury.
