What is the purpose of Prayer?
What the question says, why do people pray if God knows everything?
This is kind of like asking, "Why should you tell your Dad you love him if he already knows?" It's not about conveying information, it's about fellowship. God created us for fellowship with Him. We don't pray because God needs to be reminded of our needs, we are supposed to pray because we are to partner with Him to see His will be done. He choose not to force us to do His will, but that we are to choose to seek His will. Jesus told us to pray in this way:
Prayer recognizes God as our Father, who is holy, and seeks to put our mindset in alignment with His.
Why pray if God knows everything...
Definitions first... Omniscience means "all knowing", and this is how the Bible describes God. For example, consider the following Bible verses -
So perhaps I can paraphrase your question like this... "Since God already knows what everyone needs, why do we need to ask Him for things in prayer?"
I think it's definitely true that God already knows all our needs. Jesus says exactly that -
Still, Jesus goes on to teach the disciples how to pray, so He still thought it was vital. And James also points out the necessity of prayer when he tells Christians that -
So it seems that the Bible teaches that God knows all we need, yet He (sometimes at least) doesn't give us what we need until we ask. Why is that? I'm not sure if the Bible tells us exactly, but I believe it is because the act of praying, and then receiving answers to our prayers, actually increases our faith, and strengthens our relationship with God.