In Percy Jackson why was it important for Percy not to lose control of his powers?

In the first book, people like Grover and Annabeth talk about the children of the big three gods being more dangerous as they can lose control of their powers and in book 4 when Percy is in pain, he lets all of his powers go at once and the god Hephaestus shouts at him for losing control but why is it so important not to lose control?