If you're buying a luxury car (think BMW, Porsche etc) then it may be simply because of the cost. It also depends on how much you prioritise having a clean, glossy car. A ceramic coating is a liquid polymer used to provide a microscopic layer of paint protection and it's infused directly onto the clear coat. When applied, it seeps into the tiny imperfections found in paint clear coat to create a flat surface. Due to the tiny size of coating particles, it seals all the imperfections in the paint clear coat and makes it resistant to chemicals, UV rays, bird droppings, scratches, extreme heat etc. The flat layer of ceramic coating makes the surface very slippery, which helps to repel water, making it hydrophobic.

The transparent coating will essentially keep your car looking newer for longer, making it look glossy and easier to clean. But make sure you aren't getting it confused with a Paint Protection Film (PPF) either. They're very different products (which do a similar job and have some crossover) but a ceramic coating isn't hard like a PPF so it won't protect the car from minor damage like stone chips and the like. Some owners get both applied.

Another thing to note is that if you do want a ceramic coat applied, make sure you get it done by a professional. Some dealers will be able to do this, but some are just trying to upsell this to you in order to make more commission. If you want it done, I say go for it — but do your research into where the best place to get it done will be. Don't be tempted into something too cheap as there are a lot of cowboys about that'll do shoddy work. Also, look into what type of ceramic coating you want beforehand.