I see that a few strong GMs are playing 6.h4 vs the Najdorf Sicilian. And I have also seen this move given a dubious (?!) value. Apart from its surprise value (always important in chess), I see it as a rather logical move if you want to play aggressively vs the Najdorf. What is, in your opinion, the main idea behind this variation?
I play the Najdorf with Black and would say that on move six White has tried almost everything: almost every legal move is a move and so are attempts like 6.h4, 6.a4 or the like.
This said, there are nevertheless basic principles and ideas that make some lines more sound (and played) than others. Historically the sharpest try to refute the Najdorf has been 6.Bg5 e6 7.f4, described in hundreds of chess monographies: however, the so-called Poisoned Pawn variation with 7...Qb6 8.Qd2 followed by either 8...Qxb2 or 8...Nc6 has somehow refuted the 6.Bg5 line as it has been worked out in a draw at correct play where White cannot really force the win.
Nowadays the current trends and sharpest tests go in the direction of the English attack (6.Be3 e5 7. Nb3 Be6 8.f3) or fianchetto lines (6.g3 or 6.h3, the Adam's attack) followed by a quick g4. Here the underlying idea is, no matter what line you choose, to long castle and play g4-g5 supported by the f or h pawns (in f3 or h3); meanwhile, Black is reacting with b5-b4-Rc8 almost all the times. This means that in order to play g4 White must support its pawn with either f3 or h3 (that are currently played as main lines, almost) to avoid any chance of tactics again the g4 pawn: in this respect h4 does not defend the g-pawn, nor does it attack any of Black pieces, nor does it develop any (like 6.Be3 or 6.Be2 would, for instance). Nevertheless it is at the moment being tried at the GM level, the basic idea still being a pawn storm against the King side, but it is not yet a main line due to the fact that it does not pose any direct threat and gives Black the initiative with b5-b4 without the possibility of a quick g4-g5 (because White has wasted one move with 6.h4).