The purpose of the single slit is to ensure that the light that reaches the double slit is effectively from a point source. This first slit would be unnecessary if the laser emitting device is itself a point source. Ironically, the business part inside a laser emitting device is in fact a point source. The laser light exits the cavity via a tiny aperture. Because the aperture is so small the light diverges. With a lense this divergent light is refracted to a parallel beam. What is exiting the laser emitting device is a parallel beam with significant spatial width. If the laser emitting device is close to the double slits then the width of the parallel beam prevents visible interference effects. One solution would be to move the laser emitting device back. The bigger the distance, the more it acts as a point source with respect to the double slit. A more compact solution is to use a single slit first to ensure that the light that reaches the double slit is effectively from a point source.