These can occur in various places in and around the foot and are very similar to the sesamoids under the big toe joint. The difference between the sesamoids and these little 'accessory bones' is that they are not required for function.
Occasionally they can be mistaken for fractures or bone growths. Their presence is not dramatically problematic but if they suddenly become painful after an injury, their removal may be worthwhile.
Referred to as 'accessories' because they are not essential to foot function, and often have no purpose whatsoever, other than to cause you irritation.