Weight is good because:
Adding weight low down lowers the cars centre of gravity = more stability + more corner speed + better acceleration out of corners.
Adding weight improves the ratio of sprung to unsprung mass.
Adding weight stops you're car from doing wheelies.
When you hit a big bump the suspension always bottoms out so suspension settings are irrelevant , but a heavier car requires more energy to make it fly.
A heavy car has more inertia so will be less responsive and easier to drive, especially in bumpy conditions.
It's easier to get the correct weight distribution by adding weight than modifying the car.
Weight is bad because:
A heavy car needs more power to accelerate and uses more energy.
A heavy car has more inertia so will be less responsive and will wear the tyres faster.
A low centre of gravity can hurt straight line acceleration on a 2wd as there is less weight transfer to the rear. (Only applicable if the car won't wheely)
Heavy cars break more wishbones.
Lifting heavy objects can injure your back.
BRCA insurance does not cover cars above 20kg.