You can consider adjuvants, a wide class of materials that you add to herbicide mixtures, to be like accessories on a car. But they're certainly not the bells and whistles of a luxury model. They're more like features that make the car operate better or more effectively, like adjustable side view mirrors. A side view mirror that you adjust so you can see behind you perfectly can make the difference between pulling in front of a car, and making it home safely.
Likewise, a spray adjuvant can make the difference between getting a good to excellent kill, perhaps on a particular weed, and only getting fair to good control. Weed control specialists and event companies themselves recommend adjuvants be used with specific products.
Often the conditions determine how important it is to add the adjuvant. If weeds aren't growing, and they're stressed due to drought, it will be harder for herbicides to work. The proper adjuvant could increase the odds of getting better control if you must spray under those conditions.
Your chemical dealer can make recommendations as to what you should use with a particular herbicide to get good control, but in the end it's up to you to read the herbicide label, and see what the manufacturer recommends. Most will talk about what should or shouldn't be added with the herbicide for best results.
Timing of application and environmental conditions will also matter. In 2010 plots in the Precision Planting trial sprayed late with a postemergence spray were clean at harvest, even though they were wooly when sprayed. The farm worker who made the application believes the adjuvants he added helped pick up more weeds and deliver good control.
However, in 2011, with spraying being delayed a week later by early rains and conditions much hotter and drier when the application was made, the herbicide application, even with the same adjuvant, was not nearly as effective. Some weeds were controlled, but others were just held in check. Yield losses resulted in part of the field tied directly back to too much early competition from weeds with the corn.
That's why it's a must to factor ion environmental conditions besides just adding adjuvants. See what the label has to say about environmental conditions before deciding what to put in to the spray tank.