Proffesor Ratigan and his daredevil crew were out looking for a shop and found Ursula's instead. As soon as Ursula caught him stealing some of her food, she chased him out from her shop. Jay and Mal used to steal stuff from there. Mal once stole Ursula's seashell necklace.
According to the behind the scenes footage, it appears that Ursula's Fish and Chips will appear in Descendants 2 and might appear as Uma's pirate gang's hangout (Harry Hook appears to be next to the store during the behind the scenes).