God knows what we all need (our heart desires).God tells us in Matthew 6:8 "your father knows what you need before you ask him". But in 1 Thessalonians 5:17,God commands us to pray continually,that means we should pray always. Prayer is God's way for us to receive what we need from him (Hebrews 4:16).He tells us the reason we don't have what we need because we haven't asked or because we asked selfishly. We also need to pray because we have an enemy,the devil,who want to rob us of the good things God want to give to us.prayer is s way to stand firm against the devils tricks (Ephesians 6:18)