The shoe-laces used in Arabia at the time of the Prophet (PBUH) were different from the ones which are used in modern times. The shoes were then so designed that they could not be used without laces. It was like a strap which kept the shoe tied to the foot, and if it was broken one could not use the footwear at all. For this reason, it was ordained by the Prophet (PBUH) that one should first get the strap repaired and then use the pair of his shoes. If this was not done, one would be then wearing one shoe and leaving his other foot bare. There is no harm, however, if one has to do this under some compulsion.