1 d4 d6!

This should be more common than other lines p, I think black can equalize after this move

i started to play this but not with g6. There are some systems with d6 c6 qc7 I like it, have good results with it.

I play it sometimes.  It generally transposes into a Philidor (after 2.e4) or else into whatever this is called: 1.d4 d6 2.c4 e5.  Black need not fear the exchange variation 3.dxe5 dxe5 4.Qxd8+ Kxd8 which has good results for black in practice.

It's not a Pirc without ...g6.

1. e4 d6 or 1. d4 d6 is called the Pribyl System (among other names), per one entire book on that topic. It's quite a sound response, though until recently it had no official name. As others have pointed out, it can transpose to the Pirc or Philidor.

Can anyone explain an advantage of d6 over Nf6? 

1.d4 d6 is an interesting reply if u play Pirc or Philidor on 1.e4, and if u play KID or Old indian on 1.d4.

There is no problem with this move, and u have the option 1.d4 d6 2.c4 e5.

If u play KID and Modern/Pirc though, i don't see any reason not playing 1.d4 g6, because in some positions, black could play d5 in one move...

1...d6 is just a transposition move.

Back in the day a book called an explosive repetoire based around 1...d6 sold like hot cakes

 it's hardly an equalizer after move 1. Would you call the pirc or philidor equal? I highly doubt that...