Primitive man preserved fire because

Primitive man was probably more concerned with fire as a source of warmth and as a means of cooking food than as a source of light. Before the discovered less laborious ways of making fire, he had to preserve it and whenever he went on a journey he carried a firebrand with him. He discovered that the firebrand, from which the torch may well have developed, could be used for illumination was probably incidental to the primary purpose of preserving a flame.
Lamps too probably developed by accident. Early man may had his first conception of a lamp while watching a twing or fibre burning in the molten fat dropped from roasting carcass. All he had to do was to fashion a vessel to contain fat and float a lighted reed in it. Such lamps which were made of hollow stones or sea-shells have persisted in identical form up to quite recent times.