Well, I guess there isn't really an answer to that. My opinion is that the people who made the actual Pokémon game wanted to add another purpose or quest for us to catch 'em all. Shiny Pokémon are unique, rare, and vivid, who wouldn't want that? It is just another way for us to strive for achievement. That IS what we are made for, right?
Sincerely,
Insertname