I simply think he saw a chance to get a first-round talent for a relatively low cost. There is no grand and brilliant plan other than that and the options it creates.
1) If we re-sign Dak and he plays poorly while Lance has a great preseason, we could have Lance take over. (I consider this to be an unlikely scenario because Dak has generally put up good numbers during the regular season.)
2) If we re-sign Dak and he plays up to his regular level while Lance has a great preseason, we could trade Lance and make a profit on the acquisition.
3) If Lance plays well, we could keep him as a better backup option for the season in case of injury (and even possibly sign him longer term for that role, which I also consider unlikely if he plays well enough for some team in need of a QB to sign him).
You seem to be viewing this as if it is relative to Dak in some way. I don't think it is. The front office has been saying this whole time that it plans to extend Dak. I think Jones just saw it as a chance to get a first-rounder to possibly improve our depth or create value and see how it goes from there.