I feel that this card is to help the mermail archetype and other water-types and It also has been made perfectly to work with Bahamut Shark which effect always puzzled me since it made most xyz useless;
2 Level 4 WATER monsters
Once per turn: You can detach 1 Xyz Material from this card; Special Summon 1 Rank 3 or lower WATER Xyz Monster from your Extra Deck. This card cannot attack for the rest of this turn.
but Nightmare Shark isn't phased by this effect at all, actually it welcomes it;
2 Level 3 monsters
When this card is Special Summoned: You can attach 1 Level 3 WATER monster from your hand or side of the field to this card as an Xyz Material. Once per turn: You can detach 1 Xyz Material from this card to target 1 face-up WATER monster you control; this turn, that target can attack your opponent directly, but other monsters cannot attack this turn.
This combo can help Mermail Abyssmegalo with 2600 attack directly plus it can use it's effect as well to tribute Nightmare Shark to attack twice this turn, since Nightmare Shark will have no more overlay units and only has 2000 attack and can't even use it this turn thus is now useless so tributing it for Abyssmegalo is a good idea, so your opponent takes 5200 damage this turn directly.
I think konami figured that water-types are being under-minded as of late and that they needed a buff to help them survive this harsh need environment and give water the help it needs to maybe become meta with merlanteans
* underlined text is a link to their wiki pages