Let's go is basically the easy Pokemon game for young kids, like Pokemon Yellow was but targeted to four to five year olds. They also wanted to make something based on the mobile App. Eventually the Pokemon Go App will die and ten years for now someone will figure how to hack the game to add any Possible Pokemon you can have in Let's Go and make it appear legit because it supposedly came from Pokemon Go mobile game.
This is nothing really new. Gen 2 had the phone thing in crystal, Gen 3 had the e-reader cards; the DS games had so many event Pokemon it was insane and the 3DS games had the barcodes or whatever you call them.