Why Is This Move Inaccurate?

https://www.chess.com/live#a=165361 

I recently played this game, and afterward I did a computer analysis. It said move 12 (Re8) was inaccurate, and I just do not understand why. I don't see any move the opponent could make to undermine the value of this move. The computer says I should have played H6 first, and I'm just curious why this would be necessary or better. I would appreciate if anyone has any insight on this. Thanks.