What's the purpose of spraying water on rebar coils but not on plain coils?

The rebar may be getting some hardening depending on temperature and chemical composition. If they are hot enough to show any color ( roughly 1200 F) it is definitely some hardening. Because the transformation from high temperature austenite takes some time ( depending on chemistry ) ; some hardening could develop at lower temperatures. The rebar coils would only need to be straightened and have the anchor pattern rolled into them to be finished. The other coils are likely to have significant further processing such as wire drawing or cold heading so they need to be less hard = more ductile so are cooled slowly.