Fast road riders who want aero on Giant's chart. TCR is the lighter, more conventional sibling.
Giant's aero race bike. Stiffer at the front, more wind-cheating than TCR, still in the same size-letter family so many riders can twin the two.
Gravel riders who want Giant's drop-bar all-road, not a TCR with knobbies.
Giant's gravel bike: stable, mounts, and a position between Defy and a race gravel whip. The X variants add more suspension/travel attitude.
Gravel racers coming from a road-race fit who found Checkpoint too tall or too touring-oriented.
The gravel-race Trek: lower, longer, and more aero than Checkpoint. It wants speed on dirt, not a full frame-bag tour.