For retailers it’s almost impossible to control what’s happening in the waiting line on the street, while for customers “1.5 meters of distance” can be a bit vague. By applying the icon to the sidewalk, people know exactly where to stand and will automatically keep the right distance from each other.
For other public spaces where social distancing needs to be respected (f.e. the bus stop), anyone who wants to help out can download the stencil and chalk it on the pavement with a 1.5m distance in between each stencil (that’s about 2 big steps). Combine going for fresh air with ensuring social distancing!