They offer no support
Flat flip flops offer the feet no support and commonly have loose straps. These design flaws often leads to an abnormal walking pattern and the transfer unwanted stress and strain to the musculoskeletal system which may lead to injury.
A lack of support causes the foot to pronate like below:
When your feet pronate it has an adverse effect on other parts of your body like below:
Traditional flip flops have loose straps
Archies Flip Flops have been designed with a tighter strap than most flip flops, this is because in flip flops with a loose strap, the toes claw down in an attempt to keep the flip flops on the feet.
Normally when you walk, before your feet hit the ground, your toes are supposed to point/bend upwards. When the toes point/bend upwards, this activates the arch of the foot, which helps to lock the feet, providing protection and support to the vital structures of the feet as they contact the ground with your entire body weight.
When the toes claw down such as when trying to keep the flip flops on the feet, the arch does not activate correctly, which means that the foot is not braced for impact and you do not get the protective support from the arch like you are supposed to. This can leave the foot tired and achy, vulnerable to stress and injury. The tighter strap found in Archies Flip Flops reduces toe clawing, which enables you to walk far more naturally and allows the arch to perform its normal protective function.
'Traditional' flip flops with loose straps
Archies Flip Flops have tight straps