The 6 Move, Stealth Flying of the
Phantom Knights always pulls at me when I'm considering making an army against an unknown opponent. Thay always seem to be an option to fill a hole I'm missing in the rest of my army.
They can fill the Glyph getter roll. They can be assassins if your opponent has a lynchpin in their strategy or if they have a unit the rest of your army can't deal with.
Against squads, their extra Insubstantial 3 dice have a great potential survive the first barrage of ranged attacks and so, they often get the first kill against ranged squads or the first attack against melee squads.
They have fair point for point odds against most heroes. Even though heroes generally have more Special Attacks to bypass the PK's defense than squads do, the PK's can often hover on the edge of their threat range to close the distance for a first strike or make an initiative switch gambit (I'm genuinely worried worried to see Braxas or Hydras though)
I only have 2 squads of most commons, so splashing 2 squads of PKs usually feels more effective for a limited purpose than 2 squads of some other things which benefit more from higher numbers and sustained Order Markers.
Plus, it's just fun to flay around the board, ignore obstacles and adjacency and always get to the advantageous positions that are left open.