When someone clicks your ad and buys from your store, Google and Facebook are supposed to get notified. That signal is how they learn what kind of people buy from you, so they can show your ads to more people like them.
Your ad platform is like a salesperson who only hears about some of the deals that closed. It thinks your ads perform worse than they really do, and it spends your budget on the wrong people. You pay for that every day in weaker results.