Using light weights until you're exhausted or slow builds your muscle stamina better than heavy weights, but doesn't make you as strong as lifting heavy weights.

Doing light weight exercises until you can't do any more for 8 weeks won't make your arms stronger or more powerful if you're new to lifting — you probably need to lift heavier or move faster to see real gains.

Lifting light weights until you can't do another rep builds muscle just as well as lifting light weights until your speed slows down a lot, as long as you do the same total amount of work.

Whether you lift light weights many times or heavy weights fewer times—both can make you stronger in exercises like squats and deadlifts, as long as you push to exhaustion.

If you train with weights that change resistance throughout the movement (like using bands or machines that adjust), you can do more reps and lift more total weight before getting tired than if you use the same weight the whole time — and after 20 weeks, this difference is pretty big.

If you lift heavy weights, you get better at doing more reps with heavy weights; if you lift light weights, you get better at doing more reps with light weights — your muscles adapt specifically to the weight you use.