Magnesium is a highly flammable metal, but, while it is easy to ignite when powdered or shaved into thin strips, it is difficult to ignite in mass or bulk. On burning in air, magnesium produces a brilliant-white light that includes strong ultraviolet. Thus, magnesium powder (flash powder) was used as a source of illumination in the early days of photography.
It is usually used in applications that require LOTS of heat. Thermite (which is a combination of powdered aluminum and a metal oxide, usually red iron oxide) takes lots of heat, even more than a blow torch. The ribbon is used to ignite the thermite.