i personally believe that it's totally useless to purchase new 12 iphone, I mean, i's not kinda updated, just the design
For whom is it useless?
It's not a giant update, but Apple isn't expecting people to buy new iPhones every year (apart from the people on the upgrade program, of course). It'd be a big leap over my iPhone XS from a couple of years ago in most respects: significantly better camera quality (plus a third cam and LiDAR if it's the Pro), 5G, a faster processor, longer battery life, a tougher body, a higher-resolution and higher-contrast display... you get the idea.
If it weren't that a pandemic reduces the desire for upgrades (I'm not leaving home nearly as often), I'd have an iPhone 12 Pro.