British defeated Tipu Sultan in the Battle of Plassey. It was the first major victory of Britishers in the Indian subcontinent. It also meant eliminating an ally of France. It was the first step towards India's colonisation, after colonisation the East India Company no longer needed to bring precious metals from their country to buy Indian goods, they could use India's resources to buy Indian goods or force the Indian farmers and artisans to produce the goods for free. That's why Britishers were pleased to defeat Tipu Sultan.