Tyipical pushing back of the Bishop to g3 (or g6 often). What's so relevant about it in the end?

I was analyzing a Slav I played and then the computer (Stockfish) plays this ...h6 -> ...g5 sequence. With my puny chess understanding, the black bishop isn't that bad on on g6. Sure, it appears to do nothing at the moment, but it has available a nice open diagonal. Secondly, in addition to this attribute for black, white has ruined its king-side.

Yet this is the way to go according to the engine. I see similar things a lot! What is the thinking behind this, if you please?

I used a free Stockfish analysis from the Analysis tool.

It felt that your move was superior.

That aside, even if it were superior (quite possible) I still wonder why the ...h6 -> ...g5 maneuver is OK.