What sort of disease is needing contact lenses?

Refractive error. Contact lenses aid the eye in focusing light sharply onto the retina. This allows images to be projected onto the retina clearly, which allows us to see clearly.
Keratoconus. Keratoconus, and other corneal diseases with an irregular cornea may benefit from contact lenses as this will give clearer vision than glasses. A contact lens fits directly on the surface, so can even out irregualrities better, as long as the patient tolerates it.