The null is used to stop the macro until it gets reset, in this case due to target or the entire macro being reset by combat.(or alt which like Nathan says is not an option for me due to keybinds)
It is commonly written this way because to just write it as a /cast as you were asking about means it would continually cast (per the 4 seconds THX and Nathan were trying to avoid)
I use the same thing in my macros for spells like “Flame Shock” (on Elemental Shaman). These spells want to be cast on each target, but typically have other refresh mechanisms, or require more specific input to not waste resources and cooldowns that could be used on other spells.
While spells like consecrate and flame shock can be one button macroed to fire all the time they are off cooldown, it would be bad programming to do so and stop those of us picky enough in our parses from ever getting close to full optimization in our macros. The null option gives us some flexibility in getting fairly one button, with the option to add a little more management to get closer to the maximized end goal.