Barbells are one of the best pieces of equipment for training with. Most big compound exercise can be done with a barbell. From Deadlifts, Bench Press, Olympic Lifting, Squatting or Pressing, the barbell will allow you to take these exercises from a bare barbell up to whatever weight you need.
Most barbell exercises are compound movements (using a lot of body parts working together). The barbell is very simple tool, so it is easy to learn the basic lifts quickly.
With GMC coaching, anyone without any debilitating pre-conditions can learn most barbell exercises quickly. Olympic lifts may be an exception for some, but progressions can be made, quickly on these skills too.
The weight is fixed and stable, so the tool itself is less likely to deviate from a normal range of motion. This is an advantage for the barbell because, when under heavy load, you do not want the barbell to deviate at all. Another advantage of the barbell is that unlimited progression is possible because you can always add more weight to the bar.
Of course, every lifter has their own genetically pre-determined maxes that they can handle but being able to add more weight easily makes progressing with this tool very simple.