Upon entering the sewers, the Bros. notice Fawful walking around the sewers. If the player decides to follow him, they will be lead to his shop. As a pipe that leads to his shop is inaccessible to the grown-up Mario Bros., they send the babies instead.
Once in the shop, Fawful will recount the events of the previous game, Mario & Luigi: Superstar Saga, stating that he will rise once again and get his revenge on the Mario Bros. This is likely a foreshadowing of the events of the next game in the Mario & Luigi series, Mario & Luigi: Bowser's Inside Story. Ironically, he does not recognize the babies as past versions of the two brother he once fought and lost against.
As the shop's name implies, Fawful's Bean 'n' Badge is a Badge shop, and carries some of the most powerful badges in the game. Instead of purchasing the badges for coins, however, Fawful instead takes Beans.
If you haven't yet acessed the area you are supposed to go to next and leave the shop, Fawful "seeks a vision" and tells the baby Mario Bros. about it.