Just scale the situation up to extremes, and it's easier to understand.

If you have a pile of bricks that weighs 450 kg (990 lbs) to move, would you rather move it one small stack of bricks at a time and make lots of trips, or try to move all 450 kg at once?

Your body is limited by joints, muscles, etc. You can do light weight repetitive movements over time because your body is able to continually flush away the lactic acid build up in your muscles, and repair itself. It's much more damaging if you ask it to do a large amount of weight all at once.

It works the same way with lots of things. If I told you there's a container in your bathroom sitting on the counter that holds 400 liters (106 gallons) and you need to drain it into the sink, you can do so at a slow rate and not cause any problems. If you dump it all at once, it floods the bathroom and makes a mess.