I am reading about the King's Indian Attack (aka Barcza System), and article says that this system is "rarely used at the highest levels except to avoid certain pet lines". I wonder what are those lines, and if there is some important reason why the KIA is "rarely used"?
Those lines can be any opening variation; suppose player A has a bad record with White against player B, being defeated in some main lines which are hard to avoid otherwise. Player A may then opt to choose the King's Indian Attack, to avoid said lines.
As for why it's rarely used at top level: it is relatively easy with Black to get a playable position with even chances. White's aim is to get an advantage out of the opening, and the King's Indian Attack is not promising enough.
In his early days, Fischer would use it to avoid the French Defense, for example. It's not uncommon for e4 players to transpose into it in the face of any of the half-open defenses (French, Sicilian, Caro-Kan, etc.) they hadn't yet found a line they wanted to trust. Or for d4 players to steer away from lines they're uncomfortable playing (KIA-to-Catalan transitions come to mind in those cases).
Scenario: The variation I usually play against the French has suddenly developed a leak -- someone recently played a move against me I hadn't seen before and I haven't had the time or the ability to "solve" that move. I might then transpose (after 1. e4 e6) into a KIA, essentially "bailing out" of a situation I wasn't comfortable with to one I'm more comfortable with. This particular scenario doesn't typically happen in top-level chess, as those players are surprised less frequently, and typically have a "fallback" opening prepared in the case of their main weapon having issues.