Why should protection be implemented for steelwork against fire?

Owing to the high thermal conductivity of steel, the temperature of unprotected steel is almost the same as the temperature of fire. Since the yield strength of structural steel drops approximately by half when its temperature rises to about 550^oC, it is usually provided with some forms of insulation. The consequence of the outbreak of fire in proximity of unprotected structural steel is the potential loss of load carrying capacity of steel and the occurrence of substantial movement of steel.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.