For a third time the Israelites complain and test God about being taken out of Egypt just to die. This time it was about water. Despite all they had seen the Lord do, they hardened their hearts like Pharaoh. Psalm 95:7-9 says that is what they did.
God had Moses strike a rock for water to flow from. In 1 Corinthians 10:4, Paul says, "...For they drank from the Spiritual Rock that followed them, and the Rock was Christ." Meaning that Christ in spiritual form was with the Israelites throughout their journey.
I think the reason God had Moses strike the rock was to symbolize Christ being "struck"(crucified and resurrected) and then living water being able to flow to all that believe in Him. In John 7:37-39, it speaks on rivers of living water but they couldn't flow till Jesus was glorified. In Numbers 20, for a second time the people are quarreling about water and this time Moses is told to speak to the rock. I think it was to represent that after Jesus was glorified all we have to do to get the living water is to ask for it. That could be part of the reason why Moses got severely punished for striking the rock instead of speaking to it like instructed to do.
Joshua is mentioned for the first time in verse 17:9. He is asked to gather men to go fight and without hesitation he does just that. He had faith in God which despite what others saw and said later he remains faith to God. This is why later God will choose him to succeed Moses and lead Israel into the promised land.
For the battle, as long as Moses held the staff of God up the Israelites prevailed. This shows that when we trust God and let Him be over our lives and what we go through we will prevail.
When Moses grew weary it showed the importance of relationships that we can't do life alone. There will be times that we are weakened, having people in your life that will hold you up to God will help get you through the battle.
In Chapter 18, it has Jethro, Moses's father in law, bringing back Moses's wife and children back to him. It saying the meaning of the kids names again seems that having his kids back are a reminder to Moses that he was a sojourner and that God has been his help. God knew what was ahead and wanted to encourage him.
Jethro heard all that had happened and said, "Now I know that the Lord is greater than all gods" (v11) and then does a burnt offering to the Lord. Here is someone that didn't experience the events but after hearing of them showed more faith than some that experienced it first hand.
Jethro sees Moses taken on a larger burden than he should be and gives him the advice to share the load. As a leader it is essential to learn when and what to delegate, so no one becomes weary and burnt out.