Walking around your city, you might have noticed spray paint markings on the street and footpath. Clearly they're there to mark something, but what? Turns out, construction workers aren't just doodling for fun -- those marks are there to protect you.
The main purpose for markings in the street or on the footpath is to indicate where pipes, ducts, cables and other structures are located, and what direction they flow in, so that they aren't hit or destroyed during construction. If you've ever seen a water (or worse, sewage) pipe get busted, you know how bad it can be for you and everyone else trying to pass through the area (and of course, it's awful for the construction workers). Beyond that inconvenience, a burst service line can disrupt access to water, power and other utilities for your home or work.
In addition to helping construction workers keep underground structures intact, your city might also use spray paint markers to track services performed, like the ones San Francisco uses to confirm that a sewer has been treated with methoprene (which combats mosquitoes breeding in the drain water).