What PB said, even though there are more bepoke pieces nowadays meaning less applications for certain bricks/elements beyond their intention, the latent potential for building pretty much anything is the real appeal.
My kids get sets and build them once, dismantle them and the pieces just end up in the drawers/storage trays we've got, all mixed together. I tried sorting it all out once, so they could find the right pieces for the instructions they've got. Inevitably it all got mixed up again, but they don't care. They just build superhero bases and jails, roller coasters and racing cars. The only appeal of the sets to them is getting their favourite superhero as a mini figure.
We've given up buying sets, they're usually overpriced and either boxes of 'standard' bricks or bulk lots sold by the kilo on ebay are way better value.