Smart casting doesn't mean "It'll cast stuff for you". Smart casting means "When you hit the hotkey, it immediately casts the spell as if you were targetting where your mouse cursor is hovering".
When you hover over a unit, the colored border around it will thicken. This will denote exactly which unit you will smart-cast against when using a targetted skill (like Rammus' Puncturing Taunt), provided it's a valid target.
When using an area-of-effect or skillshot skill, then a smart-cast will be targetted as if you had clicked whereever the mouse is currently situation.
It saves you not merely time in setting up the launch, but also gives you much more flexibility in the actual gameplay. Using normal targetting practices, you have to either select your target or cancel the skill, you cannot input any other commands. With smart-casting, you can focus on the setup for your skill, and then adapt your movements to any updates in the situation without needing to cancel out. And then once you get in position, it's just one button click to fire. Since it can be thrown out from any action as fast as you can drag your mouse over, it can be a lot faster and catch your opponent off-guard, especially for otherwise predictable lead-in skills.
Its utility in being smartly used is entirely dependent on how effective you can utilize aiming without the guide ranges provided for skills. For targetted effects, it rather depends on your ability to target by hovering on a unit rather than clicking on a unit (so in clusters, it is important for knowing the timing so that you don't cast on the wrong foe). It offers speed, but unless your accuracy is up to par, it may be more effective to stick with normal casting for skills until you get the hang of it.