Why do online companies want to sell me a non-OES size tire?

My '09 Impresa Outback Sport came with 205-50-17's when I look online to replace them the initial offer is for 225-45-17's. When I hit the change tire size it acknowledges 205-50-17 as the Original Equipment Size with plenty of choices. Is there an advantage to the 45's? The 50's are cheaper to buy and cheaper to have mounted.