An offer that adds a free product on top of a purchase can be a genuine bonus, but the word "free" in this context usually depends on a set of conditions that are worth reading before it factors into a buying decision. Treating the free item as guaranteed before checking those conditions is the most common way this type of offer disappoints, and these promotions are common enough during busy shopping periods that it helps to check the same handful of details every time rather than reading each one's fine print from scratch.
Most free-with-purchase offers require a minimum spend or the purchase of a specific qualifying product, and it is easy to assume any item in a category counts when only a narrow list actually does. Checking the exact list of eligible products, and the exact minimum total after any other discounts are applied, avoids finding out only at checkout that the free item won't be added. It is also worth noting whether a discount code or an already-reduced sale price counts toward that minimum, since some offers calculate the threshold against the full, undiscounted price of the cart.
Not every free item is worth factoring into the decision at all. A free accessory that has no use for a particular buyer adds nothing to the value of the deal, no matter how it is priced on the promotional page.
Free-with-purchase promotions frequently have separate, smaller stock allocations for the bonus item than for the main product, meaning the main purchase can complete even after the free item has run out. Confirming that the free item still shows as available immediately before completing checkout is a simple way to avoid a purchase confirmation that quietly drops the bonus. It is also worth checking whether the promotion applies per order or per qualifying item, since buying two eligible products does not always mean two free items are added.
Used carefully, free-with-purchase offers can add real value on top of a purchase already being made for other reasons. The habit worth building is treating the free item as a possible bonus to verify rather than a certain part of the deal, since the conditions attached to it determine almost all of its actual worth.