Sniffing, marking, pulling, scanning, etc are all behaviours that can create loading (working itself up), anxiety and a constant state of arousal. Dogs in these states are much more likely to react, have shorter thresholds and are more likely to take action when a trigger (something that causes the dog to react/the dog reacts to) presents itself. A trigger could be another dog, person, cat, squirrel, cars, skateboards, bikes, etc.
One of the first things we teach in the process of stopping reactivity is how to walk on a loose leash (heel). Having a loose leash allows your dog to follow your lead and prevents unnecessary stress and arousal through the pressure the collar creates when the dog is pulling.
Basically, if you have a dog that constantly pulls, sniffs, scans, marks, and is control of the walks, you are creating/or have created a dog who is in a constant state of arousal.
Teaching the dog a proper “heel” and stopping these behaviours is not a solution for leash reactivity but it is definitely a step in the right direction. It will bring the arousal down and allow the dog to make better choices and have a calmer state of mind.