You can gain Life Points depending on the target monster's Attack or Defense.
It can be chained to a lot of effects that destroys a monster since it's a Quick-Play Spell Card. E.g. Chain it to your opponent's Order to Charge.
You have to tribute a monster to gain Life Points.
Tributes for cost
You gain Life Points
Why can you not use this on a monster you control with 0 atk and 0 def? Based on the activation cost of tributing your monster, I don't see why you can't unless the issue is that you literally cannot "gain" 0 life points.