Why is a bar essential for weight training?
A bar is an essential accessory for weight training, since it allows you to work all your muscle groups. Here is a (non exhaustive) overview of the muscle groups engaged.
Legs: squat (bar at the neck or clavicle), Romanian deadlift, good morning
Back: traditional or sumo deadlift, all variations of rowing (pronation and supination)
Pecs: all variations of bench press (flat, inclined, declined)
Shoulders: military press and rowing