To give us something to wonder about.

To make us question.

You may as well ask: why do polar bears exist? Or, why are there pine trees? Since you asked this question in a science forum, the scientific answer as to why there were dinosaurs is: there is no answer. It's impossible to predict how evolution acts to modify and introduce new species into the world. Thus it's impossible to say "why" any particular species exists (or existed).

If we were dinosaurs, perhaps He`d have sent us a Messenger from amogst ourselves, or we`d just be practicing our natural inclination. But we`re not, are we? And they`re dead, aren`t they? And we will not be asked what the dinosaurs used to do. And hear we are wasting our time by honouring stories of stupid scientist, who claim that we are descendants of monkeys! If you haven`t heard any saying of Allaah, or His Messenger -sAaws- explaining about dinosaurs, so THEY DON`T MATTER. May Allaah guide us.
Your Lord said in suurat Al-Mulk that He created the death and the life in order to test us, which of us perfect our deeds. And He told us in suurat At-thaarijaat (51:56): "I have only created Jinns and men, that they may worship me."
So who cares about dinosaurs? Worship Allaah, and don`t waste your life, because He created you and gave you ears to hear, eyes to see, and a mind to understand. So praise be to Allaah, and we thank Him, and seek His forgiveness, and guidance.

You are also correct. He said so in His Book: Indeed there are in the creation of the heavens and the earth, and in the changing of the night and day signs for men of understanding.

Yes, impossible unless God, Who determines the evolution of His creation has informed us.