I think I understand they this is done, but it just doesnt really click fully.

The bed is a relatively heavy object to move around, especially at higher print speeds, over 100 mm/s. The part is on the bed, and is going to go through accelerations and bounce around like a pendulum if it is a tall object, thousands of times. I find this perplexing, what exactly is the advantage of having your y-axis relative motion take place at the bed?