It is very easy to become overextended, particularly if you are talented in many areas. Moses was gifted by God to be a great leader, but he was on the path to burn-out doing everything by himself. Sometimes it takes another perspective from a trusted person like Jethro to realize this is the case. Fortunately, Moses was mature enough to recognize the wisdom of delegating some his duties, and Israel was blessed as a result (for a New Testament episode that is very similar, consider the story of the service to the widows in Acts 6).
The human body functions best when every part does its part, rather than one part of the body doing all the work. That is Paul's classic illustration of the church in today's NT reading, Ephesians 4.
And he gave the apostles, the prophets, the evangelists, the shepherds and teachers, to equip the saints for the work of ministry, for building up the body of Christ, until we all attain to the unity of the faith and of the knowledge of the Son of God, to mature manhood, to the measure of the stature of the fullness of Christ, so that we may no longer be children, tossed to and fro by the waves and carried about by every wind of doctrine, by human cunning, by craftiness in deceitful schemes. Rather, speaking the truth in love, we are to grow up in every way into him who is the head, into Christ, from whom the whole body, joined and held together by every joint with which it is equipped, when each part is working properly, makes the body grow so that it builds itself up in love (Eph. 4:11-16)"When each part is working properly" the body grows. This requires two things relative to the story of Moses in Ex. 18: every part must know its limits lest it try to take on too much; and each part must know its purpose so that it performs its own responsibility.