The two key "points" of the A318 are (a) range and (b) LESS passengers.
The A319 has about 140 pax. The A 318 will carry around 100. But, unlike most 100 seat planes, the A318 can fly quite long distances. So it's useful for what airlines call "high yield/low density" long routes - or "long and thin".
Example: No one is going to do this, but the A318 could fly from Denver to, say, Venezuela. This wouldn't be a heavily travelled route, but a small plane could maybe make money on it.
For a number of airlines with A319's or A320's in the fleet, they can substitute the smaller A318 (because of the "commonality") in low yield seasons.
Remains to be seen if the A318 sells well, rather like the B717, but a lot of people confuse them as competitors. In fact, the two planes are really designed to do different things. I wish both of them well.