Doing one heavy set plus lighter ones without resting takes less time than doing three full sets of heavy or light curls.

Doing a heavy set of arm curls followed by lighter ones without resting can make you stronger in one-rep max and static strength, just like doing only heavy curls.

Doing a heavy set followed by lighter ones without resting can help you do more reps with light weights, just like doing only light curls.

You can get stronger, bigger, and more enduring muscles by doing one tough set followed by lighter ones without resting—faster than doing multiple heavy or light sets.

Doing a tough set of arm curls followed by lighter sets without resting can make your biceps grow just as much as doing only heavy or only light curls over 8 weeks.

Whether you do similar muscle groups together or alternate them, you end up lifting about the same total weight over 8 weeks.