Ask questions in the Help thread...assuimg that you're asking a non-rhetoric question. =P
But they're just there...because the programmers decided to put them in, really. =P Might be because they aren't obtainable in this gen alone, maybe they thought it'd be something extra and fun for the player to do, etc - and that's it. Note that getting both (i.e. trading) unlocks Rayquazza too, so there's another in the 'give something extra for the player to do' department.
Anyways, thread closed - answered and I don't see much discussion coming from this. Same could be said about Lugia and Ho-oh, or Mewtwo, etc, etc. =P