4... h6 is a multi-purpose waiting move. In the main line, Black plays 4... Bf5  and is then chased to g6 by 5. Ng3, after which White will plays h4 and then h5 to harass the Bishop even more and force Black to play h6 and Bh7.

In the Finnish Variation, Black has already played h6, so the Bishop can move to h7 in one go and avoid further harassment. Also, by postponing Bf5, Black obtains the extra option of playing 5... Bg4 if White plays 5. Nf3. Lastly, 4... h6 stops White from playing Ng5 or Bg5.

why h6

when you can win without it

4.Qc7? instead of h6? because number shows that White is a bit better after h6?

Got it.

It prevents Ng5 in response to ...Nd7; see: http://www.chess.com/opening/eco/B17_Caro_Kann_Defense_Karpov_Variation_Modern_Variation

Im not trying to be contrary here, but what does moving that pawn accomplish?