The Dimensional Box was added in Beta 8.3, but was already changed in Beta 8.3.2, a few hours later. It catches a monster of your choice when placed, including your own. But once this card dies, the monster you caught will return to the board, if possible.
This monster can be useful to more or less permanently paralyze an enemy monster, especially when combined with Taunts to keep the Box alive. You can then kill it when the enemy board is full, as that will kill the caught card. It can also be used against an opponent with Memorial Statue to store another monster, have this card silenced, and not give back the monster. You can also use Break (in Integrity) or Doggo (in general) after placing the Box, as that will also remove the caught monster from play.
Just like with cards like Lamp and Igloo, the released monster will not activate its Magic or Future effect.
Keep in mind that the Dimensional Box stores the monsters and not kills them, so they won't increase Chara's damage.
Since it can catch your own monsters and releases them when it's killed, It can be used to save one of your own cards when using Hyper Goner.