Lip bumper is used for:
Mouth breathing
Nail biting
For lip biting & hyperactive rnentalis
All of the above
Lip Bumper
Lip bumper is an appliance that makes use of the tonicity of the lip. The lip trap and excessive lip activity can be helped with a lip bumper  which prevents lip pressure on the lower incisor; their labial position gets normalised with tongue pressure. The lost arch length can also be gained with this method.
Key Concept: 
The lip trap and excessive lip activity can be helped with a lip bumper which prevents lip pressure on the lower incisor.
Reference: Orthodontics: Diagnosis and Management of Malocclusion and
Dentofacial Deformities 3rd ed Om P. Kharbanda pdf no 736