This is because the merchants want more customers to buy things, so the price is cheaper. A 15% discount means that the current price is 85% of the original price. A 10% discount means a 10% discount, which means that the current price is 10% off the original price.
I went into the supermarket and got a bag of potato chips. I read that it's 20% more than before, and 20% is 20%, which means that the grams now are 120% of the original grams. I took another bag of milk, and it was written on the back that protein was greater than or equal to 3%, that is to say, the grams of protein accounted for 3% of the whole bag of milk, or greater than or equal to 3%.
On our way to grandma's house after shopping, I think that percentage is really useful! You can use so many places, such as potato chips, water, drinks and so on.