Just like in chess, you cant win by evaluating your play on a situation by situation basis... You must see a pattern and instinctively know what the next move is...
Canned lines are the same. When a situation presents itself, a pattern will emerge that allows you to fire of these lines that you remember. If you haven't used it many times, then obviously its not gonna sound too fresh... with practice in the field, you can pass them of as being original and ad-lib...
Canned lines will give a newbie a rocket propelled start into the world of pickups without them having to rack their brains trying to think up something they have no idea will work or not...