Exo 17:11 Whenever Moses held up his hand, Israel prevailed, and whenever he lowered his hand, Amalek prevailed.
To impress upon Israel the importance of intercessory prayer, God had Moses on top of the mountain raising his hands in prayer. As long as his hands were raised, Israel was winning. The moment his hands went down, Israel was losing. Both Aaron and Hur rather than join Joshua in the battlefield understood this move of God that they assisted Moses in keeping his hands raised.
God made sure Israel understood that their success was not on their skills and ability but on God in prayer. This was to demonstrate that what we have must be empowered by strong prayer for it to succeed.
The truth that we must grapple with is that when we work we work, when we pray God works, heaven goes to motion “For bodily exercise profits a little…”, 1Tim 4:8, but “…The effective, fervent prayer of a righteous man avails much”, Jam 5:16.
To convince Israel that the hand of Moses (with whom they had just now been chiding) contributed more to their safety than their own hands, his rod than their sword, the success rises and falls as Moses lifts up or lets down his hands. Perhaps we need more leaders leading from the mountain top in prayers than in field in their owns strength.