Here is an example. It is impossible to get a shiny victini unless you used some kind of hacking system. Why did game freak make a shiny version of Victini and several other Pokemon if it's impossible to get one without hacking?
Two reasons for this. Of course, only Game Freak knows the answer, though we can still speculate.
This is in case hackers decide to make a shiny form themselves.
I actually asked this question once - they just do this so if hackers try to glitch up the game by attempting to get a Shiny Zekrom or something, then there is a shiny form already available for them to make sure that the game doesn't get ruined.
Hope I helped. :)