When you pick up a cosmetic product, the first thing that catches your eye is the front of the package. "Firmness & Elasticity," "Hydrating Serum," "Contains Collagen" -- alluring words line the display. But these are all marketing claims. They are copy crafted to present the product in the best possible light within the bounds of the law.
Front = Marketing, Back = Facts
In contrast, the "Full Ingredient List" on the back is factual information required by law. Every ingredient is listed in descending order of concentration. It cannot contain falsehoods.
In short, front = marketing, back = facts. As an extreme example, a product loudly proclaiming "Collagen-Rich Serum!" may not show "Hydrolyzed Collagen" until the 20th position on its ingredient list.
Simply building the habit of reading the back before buying can save you from unnecessary spending.