The three most important functions of any shoe inserts, whether for high heels or for flats are to add comfort by:
1. Cushioning the ball of the foot
2. Stopping the foot from sliding forwards, or down the slope in high heels
3. Stablizing the foot from moving about inside footwear during vigourous movements such as in playing sports or running, and stablizing high heels from wobbling which makes walking naturally difficult
Inside running shoes, such as the shoe illustrated, and other footwear that are used in active sports the forefoot insert acts to stop the foot from moving forward inside the toe box of the shoe and prevents bumping and consequent bruising of the toes from the toes being pushed into the front of the shoe while jogging or running. Some runners prefer shoes with plenty of space for toes to spread out, but when wearing this type of runners the laces have to be tied tight to keep the foot from sliding forward and crushing the toes into the toe box. If the shoes are laced loosely the benefit of the roomy toe box is lost if the toes crowd forward in the runners.
In hiking boots, especially when walking on trails where there are downhill parts the foot is prevented from pushing into the front of the boot by the inserts. The foot is held firmly in place by a soft cushion in front of the ball of the foot but the toes are free to function for natural walking. The laces of the boot do not have to be tied extra tight so that the foot will stay in place. This is especially helpful for people who have narrow feet but are wearing average width hiking boots or prefer boots with a lot of wiggle room for toes.
In both runners and hiking boots the feet will benefit from having a shock absorbing cushion positioned directly under the middle of the ball of the foot. This adds targeted comfort during running or hiking long distances without crowding the shoe or boot.
Other footwear, such as thong sandals can also be painful to your feet if your feet slide forwards. In thong sandals the forefoot part of the insert cushions the foot underneath and the toe grip keeps the foot from sliding forwards with a soft ridge that fits under the middle three toes. By stopping the foot from sliding forward the shoe insert stops the tender area between the toes from being pushed into the post of the thong sandal. An added bonus is cushioning under the ball of the foot that stabilizes thong sandals and keeps them on your feet ! Thong sandals aren't called "flip-flops" for nothing.