I think CheeseHead87 hit on it except I don't think he has much of a chance to make the roster, let alone an impact. But why not bring him in? At worst, they bring him in and gather more information about him, which may be useful at some point in the future. He'll likely compete with other WRs for the Practice Squad and if he doesn't make it, they're out nothing. He would be just one of 37 players currently on the team who aren't going to make the active roster. Thompson is more active than most GMs in shuffling players in and out - even during the season - in order to keep up to date with information on players. Compared to their revenue the cost of air fare and their per-diem is almost nothing.