Different brands come with different shoe sizes, so try out various brands to figure out which is the best fit for you (and your feet!).
They fill any gap between shoe and feet while adding comfy cushioning.
They make the shoes just that much smaller, by adding cushioning to secure the foot and prevent any excessive movement such as sliding.
The stickiness of the tape on both your foot and shoes allows for it to secure your heel bottoms to the shoes, adding grip.
Your foot will not slide forward or lift up with every step, when the front of the shoe is filled.