Bottom line: "why" = $$$$
Breeders make bank. As long as people are willing to spend thousands of dollars on a "designer" mix, others will continue to produce them. Not the reputable breeders don't make $$, but at least they put most of that $$ back into their breeding program (health testing, show/titles, etc).
People will continue to buy these dogs for a number of reasons. Ignorance and hype - "next new breed", "highly popular new breed", "best of both breeds", "hypoallergenic". Status - "it cost thousands so it must be worth a lot, if i have one that means i make a lot of money - everyone will see that and think 'wow'". Television. Word of mouth. "If this star has it, or it was featured in this big movie, then it must be the "in" thing."
There is no conceivable reason why some of these mixes are being produced. None. I mean, why Husky/Poodle? Why Bernese/Poodle? Why Pug/Beagle or Dachshund/Chihuahua? They serve NO purpose. And some of the traits contradict one another. Genetic disasters, is what they all are.
From what i remember, the original doodles were a project started by someone looking to create a better service/working dog. What he was hoping to create that he couldn't find in another recognized breed, i don't know. But he failed to produce the traits he wanted in any kind of predictable or repeatable way so he gave up the project. He has since spoken out against the doodle/oodle/poo breeding craze and regrets that he started it, even if he did so for an actual legit reason (though again, what traits he was looking for and why he couldn't find them in an actual recognized breed i don't know.)
So... there was an original purpose, of sorts. Now it's mainly profit. Status. Ignorance.