Used to be they were recommended for expanding the chest of those whose bones are still growing. You would lift into the pullover and take a deep breath at the same time. This is supposed to expand the rib cartilage for a larger and deeper chest.
The classic way to do a dumbell pullover is by lying crossways across a weight bench with your upper back supported, then grasping the dumbell securely in both hands and the elbows slightly bent, lower the weight and raise it to a position a little short of vertical.
I like to use a delined bench and a triceps curl bar. Or a light dumbbell held in one hand as I lie in a bed or on a sofa. I may lift a light weight for high reps, 50-100 per hand. It's nice to feel the pull of muscles on the ribs as I do this.
And yes, the exercise tones the triceps and lat especially up high near the armpit, where a lot of us tend to sag.