Some individuals keep forgetting to wear their retainers after orthodontic correction. They prefer to have something ‘permanent’ so they don’t have to bother putting on and taking off a removable retainer at night.
A lingual retainer is a piece of wire bonded behind the teeth with cement. They can also be used to stabilize shaky teeth, e.g. those teeth with severe gum disease. They are not visible when looking at the teeth from the front.