You probably assumed that stores weren't giving away free samples just to be nice. However, the real mechanics of how free samples make you buy more stuff are pretty subtle and interesting. No one does samples better than Costco, whose legendary food sample days give you more than enough energy to push your oversized cart of oversized goods through the oversized store and to the checkout (cha-ching). It's not just that trying something gives you more knowledge about what you're getting if you buy. As The Atlantic explores in a recent piece, it's that the mere act of sampling itself makes you a more receptive purchaser.
Basically, humans are extremely social creatures who are prone to taking cues from others, and who generally want to reciprocate when others do something for us. In the absence of some good reason not to do unto others how they have done unto you, people dish up what they receive in a reciprocal exchange most of the time. What this means in the store sampling case is that once the store's friendly agent has given something, you feel the somewhat silly need to give back by way of a purchase — and you'd feel like a moocher if you didn't. (Hint: this is also why charities send you unsolicited custom-printed mailing address labels in the hopes that you'll reciprocate with a donation).
Interestingly, samples are more effective at inducing purchases amongst less-educated consumers. While Costco caters to a middle- to upper-class clientele (people who can afford memberships, and to stock up on large amounts of items at a time), this may explain the sampling you see of cookie fragments and diced hors d'oeuvres in regular chain grocery stores nationwide.
There may be an advantage to simplicity in sample displays, because famous "paradox of choice" research suggests that customers offered too many choices become less likely to buy due to indecisiveness. And part of why samples work is because there are people around to provide light social pressure — those sample ladies in hair nets aren't just supervising the stash, but chatting you up and drawing an interesting, impulse purchase-happy crowd too.
I'm constantly amazed by how many people will line up for other free stuff, like free ice cream or doughnut days that various chains offer, but I guess the joke's on me to be surprised anymore by this actually totally predictable human behavior. It is quite a sight to see well-off university students standing patiently around the block from the Morningside Heights Ben & Jerry's for a free cone worth perhaps $4. But big brands want brand loyalty, and they're willing to pay for it, even in efforts that build amorphous, hard-to-measure sentiments as a return on investment instead of immediate additional spending at the cash register that day.
You'll always get a few unscrupulous customers taking advantage of free sample afternoon by stuffing their faces (and even pockets) with goodies, or someone who games the system. But companies with enormous marketing budgets and the manpower to figure out whether their efforts are working simply wouldn't do samples if they didn't make financial sense.
Images: Milles Studio/Fotolia; Giphy