6% may not sound like a lot but they also sell a fucktonne of shirts. Also I may be wrong but I believe money for name prints goes to the club, so signing a player like Beckham where everyone will want their name on the back of their shirt for 10 pound each or whatever adds up.
Example:
Manchester United apparently sold 1,750,000 million shirts in the 2015/16 season, at 60 pound a pop that adds up to GBP105,000,000, off that you're getting about 8 million on shirt sales before printing etc so it can help a lot.