Cortana, siri and Google now are the future of advertising and sales. "Okay Google, order me a pizza". "Hey siri, call me a cab". "Cortana, I need restaurant reservations at 8. Something nice".
The virtual assistant's job is to pick a supplier and spend your money on your behalf. Everybody wants a piece of that.
Maybe - but users won't go for it if there's a principal-agent problem. Google and Apple will face a temptation to choose suppliers that pay the highest commissions and they'll have to overcome that temptation if they want their services to take off.
They won't get users to say "Siri, book me a flight" if it books you a $500 American Airlines flight when you wanted a $300 Southwest flight.
The virtual assistant's job is to pick a supplier and spend your money on your behalf. Everybody wants a piece of that.