Numbers 10:9–10 (NASB) tells the Lord's people to do certain things to remind God of their existence or need of help in battle. Why should God need to be reminded?
God didn't need to be reminded. He wanted to be remembered by His people, and to be sought by them—for their welfare, not His. But they needed memory aids such as the blowing of trumpets to focus attention on their present peril, and to encourage united prayer. It was they who needed reminding (a) that the Lord knew what they needed and (b) that He was disposed to help them, if only they would ask for it. He knows what we need before we ask, but He wants us to pray, to remind ourselves that our source of blessing is the living God.