It creates character, energy, attitude, weight, lead and follow. It creates physical and emotional performance. Timing is used to show effort in thought and movement.
When a child is sneaking up on a butterfly, they move very slowly, struggling to maintain balance until the moment of attack. There is great effort in moving slowly, especially when taking long strides. A slow sneak is contained excitement showing physical and emotional force. The attack is released excitement and physical energy exploding into a fast step or lunge to catch the butterfly.
A tired or sick person will walk pained and slow unless running for the bathroom; quickly shuffling for the bathroom means something different. An excited toddler will walk fast to keep up with their long legged parents. Timing and posture will show age, and it is very important in all animation.