You may think that we're getting something for nothing - an extra light on full brightness. However, there is a cost: the battery will run down in half the time. This is because it has to supply twice the current that it did before.
Picture 2.5. What happens to the current in a parallel circuit?
What happens to the current?
When we connect one lamp to the battery it takes a current of 0.4 amps from the battery. When we connect the second lamp in parallel, it comes on with the same brightness. So it is also taking a current of 0.4 amps. But now what is the current from the battery?
At points X and Y, the current adds up. So the current being taken from the battery is 0.8 amps i.e. twice as much as it was before.
You can see that the two currents flow parallel to one another.