Their roles are to comment on the battles happening in tournaments and to introduce the beybladers before Beyblade matches (Blader DJ does all of this by himself in the Japanese version).
Brad Best: If Tyson doesn't bring home the bacon and cook it on the grill, the BBA Revolution will be left out in the cold when the cows come home to roost! A.J. Topper: Uh... yeah
Brad Best: Hello, fans! Our last match in Cairo, Egypt is about to begin. A.J. Topper: Good thing! I've got sunburn on my heat rash! Brad Best: Ouch!
The dubbers probably added them in the English version because sports are often reviewed live by two people. This was most likely done to appeal to American audiences and is one of several examples of "Americanization" in the dub.
Dizzi was also another example of adding a new character to the original.