I am thinking that Frost doesn't want to make the same mistake that Riley made by not getting the QBs that you need to run your offense in your first recruiting class. He also probably knows that injuries happen and can ruin a good season, se he needs backups.
Also, maybe he is not getting the vibes he wants from some of the QBs coming for visits.
Another thought is that some of these new QBs that are coming for visits over the next few weeks are listed as athletes and have a chance to play other positions if they do not work out at QB.
Better safe than sorry and a good idea to cover all of your bases just in case the current QBs on the roster decide to up and leave anyways.
They want one more QB. Will give out multiple offers and after they get one to accept they will rescind all the other offers. It's called parallel recruiting vs. serial recruiting.
Fair assumption is that they want to get one more guy and redshirt. As we all know, we don’t want to get ourselves in a situation where a walk on has to take the snaps. As for Barrett, his film is impressive.