Eagle is a radar and sonar plane that can locate land and underwater units.
It has a decent radar range and speed so it's the best unit you can use for scouting. Put a few in your fighterwall to have a permanent overview of the front, even when you lose the radars on land.
Always send 1 or 2 with your bombing runs to locate targets for your strikeforce. When hunting for a commander, the radar plane can locate it even if it’s cloaked. Its sonar ability can also be used to detect submarines and amphibs.