I didn't start using smart-casting until early S4. Hell, I didn't even use a mouse until S3 (was instead using a track-pad).
Smart-casting is, simply put, casting the spell without needing to click again to confirm it, on the location of where your mouse is. It's faster. Most directional skill shots will fire automatically, but those that require to be in range will make the champion walk until {s}he is in range. For targeted spells, your mouse must be over your intended target; if your mouse isn't on a valid target, then the spell simply won't activate. Just think of it as auto-clicking, so to speak, when you hit the QWERDF/1234567.