Weights: why do sets?

You do sets to work your muscles to exhaustion. The theory is this: your muscles get stronger based on repeated trauma. Every lift frays them a little, like they were ropes. To get the most out of your workout, you should be giving it all you have. As a consequence of all this, the only part of a workout where you are actually accomplishing anything is the last few reps in a set. Multiple sets make sure to wring every last drop of effort from your muscles, encouraging them to grow faster.

Example: You're on the bench, getting ready to bench-press. You do four sets, lets say. First your warm-up set at half your max for ten reps. Then a set of eight reps at 70 percent. Then a set of six reps at 75, then a last set of four at 80. All four of those last reps will be difficult, you'll have to strain a little. Or a lot, as you move up in strength and are lifting more.

The way I was taught, if you were working on muscular endurance, you did lots of reps with a low percentage of your max (think twenty or thirty reps per set), and if you were working on strength, you did fewer reps as in my example above. The intermediate goal for strength training was to get to the point where you were lifting your max for a set of four: then you would retest, establish a new max by doing sets of one rep and adding weight each set until you couldn't lift it, and start all over again. The actual workouts tended to be more in increments of 2 or 3 percent rather than 5, as I used them, but the principle is the same.

because u have to balance it out. and no reps is how many times u can do it ( ex.curl) in each set

if you do it in sets you lessen the risk of hurting your self by injuring a muscle and it also helps you do more reps overall than if you were to just go all out and burn all of your energy

and yes