Any action is completed by multiple muscles, but different actions have different emphases. At the beginning of light weight training, the effect of muscle coordination may not be obvious, but with the deepening of training and the increase of weight and strength, the coordination of various muscle groups will be more and more needed.

If you only practice your arms alone, you will find that after a period of practice, your arms will not grow and your weight will not go up, because the current training weight is not strong enough for your arms, but it is beyond your ability for other muscle groups.