The case for prebiotics is completely different. Prebiotics are non-digestible food compounds that reach the intestine largely unchanged which makes probiotics extremely happy since they feed on them. Starch is prebiotic that is fermented by probiotics.
The good bacteria in the gut eat the same foods as your dog but in order to thrive they need this prebiotics which can also be administered as dietary supplements.
In summary your dog needs prebiotics for the probiotics in his gut to thrive.