Smartly using smart casting
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.
Smartcasting is "just being able to cast your abilities on where your mouse is". Smartcasting might make a difference in laning/ teamfights. Smartcasting is typical for mid lane casters. There are champions such as Ryze, Cassiopeia, Veigar and akali that really profit from smartcasting spells. Especially veigar because you need to dish out all your spells during the period of your stun. Also the typical Deathfire grasp adds another spell that needs to be fired within the stun time. Also in laning it is of advantage if you can instantaneously dishout your combo and get out of your enemies range before he can react. I personally run smartcasts on all my spells on all my champions~~ yet i do not recommend this. Im not used to not using them anymore, this makes it difficult for me to learn new champions with different ranges and types of spells. I HIGHLY recommend using Self+smartcast on certain champions such as Morgana (primarily on solo lanes). Generally mid is an often ganked lane, because bottom is usually well warded and top is a tricky lane to gank with champions such as kennen or riven on them. So that means when a gank happens its a HUGE advantage if you can just click a defensive spell whileeee using your mouse to direct a snare/stun at the ganker or enemy laner. I hope that helped :)