If god will do his will whether you pray or not, what is the purpose of prayer?

if god is all knowing, you shouldnt need to pray for it anyway, god should already know what you want or need, he will only provide it if he wanted to anyway. also, if prayer and faith healing worked, why does the population records indicate that there were much less people before scientific progress. shouldnt prayer be just as effective in the absence of scientific progress? why did so many children die of easily prebentable diseases before childhood vaccination?