The few minutes right after milking are when a teat's natural defenses are most vulnerable — the streak canal (the small opening at the tip of the teat) hasn't fully closed yet, which is exactly the window when bacteria can most easily enter and cause mastitis. Applying disinfectant during that window is one of the best-evidenced, lowest-cost interventions in dairy hygiene, and doing it consistently, every animal, every time, matters more than which specific method you use.
A spray gun makes that consistency easier to maintain than manual dip cups, especially once you're working through a herd of any real size — it's faster per animal and doesn't require refilling or repositioning a cup for each teat individually. It's a hygiene tool rather than a mechanical wear part, so unlike most of the products in this category, it's not something that "breaks" so much as it's a piece of equipment worth having as standard practice in your daily routine.