This card can be useful when a Field Spell Card is face-up, since most monsters will have ATK higher than their original ATK (though it depends on whether or not your opponent's monsters are affected).
This card is a good card against a "Gravekeeper's" Deck, because "Necrovalley" increases the ATK of all "Gravekeeper's" monsters.
Note that this card states that the equipped monster's original ATK must be lower than an opposing monster's current ATK to use this card's effect, so you can gain some serious points by using it on a monster you control whose current ATK is higher than normal (ie. Equip Spell Cards), then gain additional ATK from this card's effect. This can be very useful in toppling powerful monsters and dealing some impressive damage.