Patriots' "draft after the draft" strategy is actually pretty simple. Get some NFL tape on some guys that are developmental, but show productivity in the skillsets they're looking for. This allows them a lot of cap flexibility while letting other teams basically be their farm system. Vrabel. Nink. Hogan. There are other examples throughout the tenure.
Additionally, you do this at the end of the roster where they can prove out in practice. This means a wider cap based across the entire roster instead of invested highly in every front line position (Khalil Mack says hello). This is highly frustrating to stat guys who want to see big numbers at every 'sexy' position, but as a long term strategy usually gives the team an ability to maneuver and test these additional "draft picks" for lack of a better term.
And yes, many/most do not pan out, but it's so low cost and risk that the hits are well worth the high number of misses.