Malignant tumors do a number of things that make them dangerous.

1) They reproduce at a fast rate

2) They create their own blood vessels to feed their obscene growth

3) they spread throughout the body by shedding cells from their origin point, and the cells travel throughout the body through the blood stream or lymphatic system.

4) they invade other tissue, which can destroy the tissue, whether it be bone, an organ, or something else, like a microscopic seed that plants in a sidewalk crack and eventually grows and breaks the sidewalk into many pieces

Benign tumors do none of these things. They do grow, but much more slowly than malignant tumors. They can't create their own blood vessels, which means the amount of nourishment they receive is lower. They don't spread anywhere, but stay where they began. And they won't grow into and destroy other body parts.

That doesn't mean they're entirely harmless. For example, benign brain tumors can be quite dangerous, because they grow and expand but the skull will not change size or shape, so instead, the benign tumor starts pushing on the brain, which can destroy parts of the brain and kill you. But benign tumors are in general much more treatable than cancers. Because they don't invade other tissues, they're easier to remove. They're kind of like a marble you drop in some dough. You can reach in and pluck it out, just having to move the dough aside a bit. But a malignant tumor may have grown all throughout body parts, so that you can't separate the tumor from the healthy tissue. Kind of like trying to remove a plant from your garden, roots and all, without disturbing any of the dirt.