When a Griffin attacked Camelot, Arthur and his knights went our riding to defeat it. They used lances while fighting. But Merlin and Lancelot worked together in order to defeat the Griffin themselves after Arthur failed and was dismounted of his horse. Lancelot then came in and defeated the Griffin himself (with Merlin's magic) with a lance (Lancelot).
When Camelot held a jousting tournament, knights used lances to compete in the joust. When the assassin Myror killed Sir Alynor to get into the joust himself in order to kill Arthur, he modified a lance so it could point out a spike. He was successful in getting into the joust, but ultimately failed after Merlin's magic tore him off his horse (The Once and Future Queen).
When Arthur, Sir Leon, Merlin, and the Knights of Camelot went out riding to make a last attempt on killing the Great Dragon, they used lances. But they were all dismounted off their horses and most of them burnt in the dragon's flames. When Arthur made one last attempt to kill him with a lance off his horse, he struck one heavy blow but the dragon easily pushed him away. Soon after that, Merlin stepped in and commanded the dragon with his Dragonlord abilities to stop attacking Camelot (The Last Dragonlord).