Addressing the original topic:
Why buy a mahogany chess set instead of a plastic one? Both sets have the same rules and there's no difference in play.
It's a matter of class and eloquence. You can have fun with a plastic set, I've played with plastic chess pieces (and go stones for that matter) all the time. When you want a touch of class, to use those pieces of equipment that have, through thousands of years evolved to be the apex of a hugely influential cultural game, then that's a reason to buy Slate and Shell. To play on a Kaya board with quality stones adds a depth to the game you can't get with normal glass or plastic, this depth doesn't come from the game itself but the added aesthetic feeling you get when you enjoy these pieces. It's hard to describe

Perhaps it's the "soul" of the items that adds to the quality. Imagine buying a Katana, hand forged from the finest Tamahagane iron sand; carefully crafted and polished by artisans over weeks or even months of work. You can buy a sword of equal cutting quality from a cheap factory in China for a mere fraction of the cost, but it's a souless, cold thing when compared to something that has actual care and love go into the manufacture thereof.
Slate and shell stones go through a careful manufacturing process where each individual stone is checked for flaws and some sets are even entirely hand made (Usually the highest quality native Suwabate stones). Each piece is in it's own right a work of art (as much from the clam as the manufacturers) whereas plastic, glass and (I hate to say it) Yunzi don't quite have the same intensive manufacturing process.
Another example, why buy a grand piano when you can get a small electric one at a fraction of the price. Both play just as well, but which looks better in the home? The grand piano adds a great sense of class and eloquence to any living room and while the electric piano is practical it just doesn't hold up in comparison.
It's a personal decision I suppose, but I recommend having a cheap go set around even if you do purchase an expensive one.
As a side note:
Slate and Shell stones don't chip nearly as easily as glass (especially with the higher grade shell stones) and feel much better than plastic.
