It's not about changing God's mind. Humans have free will to do good or to do evil. Even though God is outside of time, and in essence already knows what is going to happen, humanity within time is allowed to participate in defeating evil. One of those ways is the offering of the act of prayer. Often we don't physically engage evil but the act of prayer allows the dispensation of grace which curbs many evils intended in the world. Everyone can fight in the battle.