I often roll hp so my players can't determine exactly when a creature will fall. So when they are attacked by multiple of the same creature, they can't just count hp and know "Oh, that one has only one hp left, let the weak guy get it." Also, I use DM's Familiar and it has the option to roll random, use the average, or use specified values so it's very easy.
Also, if I want to scale an encounter slightly, I'll just use HP to do it.
Also, if I want to scale an encounter slightly, I'll just use HP to do it.