If you use "en casa" it means at home, and i thing that is what you want to say. But sometimes you want to be specific, and for some reason you want to use 'en la casa'. For instance, you are travelling and you rent a house with some friends, and someon ask you where somebody is, so in this case you would say 'mi amigo esta en la casa' and not 'mi amigo esta en casa', because 'mi amigo esta en casa' means at home .... at his home, and in this particulary example he is in the rented house, so it would be correct to say 'mi amigo esta en la casa' (my friend is in the house)