The thickness isn't the main benefit of the aftermarket recoil lugs.
The main benefit, and the one that makes the $35 investment worth it is that they are precisely surface ground so the two faces are exactly parallel. Just like Broncbob pointed out.
Does it make a discernible difference in accuracy? Very likely none that most humans would be able to notice but when the barrel and the action are screwed together without stress points here and there it can't be a bad thing.
Besides, they look so much better than the punched out factory lugs that this alone makes it worth the small investment.