The truss rod is there to do one thing—to keep the neck of your guitar straight and stable, keeping your instrument in tune all the way up its well-aligned neck. It does this job well with ingenious simplicity and efficiency, constantly counteracting the tremendous physical forces that continually conspire to bend, warp and bow the neck, preventing proper intonation.
If you’ve ever wondered about truss rods, Fender has posted some details about the history and functionality of truss rods.