How benefitial is weight training to any exercise/weight loss regimen?

Can you answer chrstnwrtr’s question about Bodybuilding?:

My dad says it builds muscle and stuff. Is weight training (machines and free weights) imperative to any exercise regimen?

For the record, I used to be able to struggle lifting heavy stuff around 30 lbs.; now I can lift 30-35 lbs. with ease.

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