It's a team-building tactic Bill Belichick learned from NFL coaching legend Chuck Noll and his Super Bowl winning Pittsburgh Steelers, however it will have to be put on the shelf from now on. According to Mike Reiss of ESPN, the NFL has now made it illegal for teams to practice with jerseys that do not have numbers on them.
The thought process behind wearing numberless jerseys is so communication between teammates during practice progresses.NFL players and teams respond to Orlando Shooting tragedy Start SlideShow
According to NFL spokesman Brian McCarthy, the rule was created to "ensure clubs are in compliance with workplace rules," and that this "will permit the league office and NFLPA to monitor player participation during on-field sessions.”
Source : https://247sports.com/nfl/new-england-patriots/Bolt/NFL-forces-New-England-Patriots-to-wear-jerseys-with-numbers-in-practice-45769861