Let's talk about brand advertising, using cars as an example because it's pretty clear cut.
Even a high spender on cars is only going to walk into a dealership and walk out with a brand new car a few times in their life. So why are we bombarded with so many car ads, all the time, across all media? Because cars are big ticket items and the choice of which dealership you walk into is critical. Car ads work to establish certain qualities in your mind as associated with the brand. VWs are reliable, Porsches are well-engineered, BMWs come from a rich motorsport heritage, etc etc. If you share those values, and if you want to broadcast those values to the world, you gravitate towards the brand that matches. No car ad is designed to make you go and buy a car then and there; that's simply not a thing that happens. They are designed to add up, over time, to have a subconscious effect on you, and influence your future decisions. And they do.
John Lewis's Christmas ad is exactly this. It's not supposed to make you do your Christmas shop at John Lewis. It's supposed to make you associate a blend of desirable Christmassy things -- kindness, generosity, warmth, family, wholesomeness -- with John Lewis. And it's amazing at it! The release of the ad is now an annual event. People talk about it. Soon ,or maybe already, if there was no ad, someone, somewhere would say "it doesn't feel like Christmas without it." And ding ding ding -- that's gold dust to advertisers. To create links in people's mind between the shop you run and an event that is so positive is amazing stuff.