The idea is to have an advantage over your opponent. Some champions (Draven, Riven, etc.) thrive off of being aggressive and enjoy snowballing a lead, however almost everyone has core builds and powerspikes. Imagine Blitzcrank/Jinx vs. Thresh/Ashe: Both Jinx and Ashe have 0 mobility in regards to "dashes" (like Lucian).
When is the lane most dangerous for Ashe? When Blitzcrank has his Q. Hence why you'll stand behind your own minions and do your best to avoid being hooked as it is (without summoner's up) a certain death.
Now look at Jinx's perspective. When is the lane most dangerous for Jinx? When Thresh has his Q. Same as above, hide behind minions.
What happens if Blitzcrank hooks a minion? The Thresh/Ashe duo lane will now IMMEDIATELY move forward knowing Blitzcrank doesn't have hook for another 20-16 (dependent on level and CDR) seconds. Without that crucial cooldown, their kill potential is absolutely diminished - especially if Thresh hasn't thrown his Q yet. The Ashe now has complete control over the lane and minions until Blitzcrank has his hook back up. If they try to trade without their spells, Thresh can easily hook one of them since they are past their minions and therefor have no safety net to dodge with. The Blitzcrank/Jinx will now stand under tower and lose valuable experience and gold, putting them further behind. This all started because Blitzcrank missed his Q, and this could all be avoided by simply NOT using the aforementioned spell. By having the threat of "what could come" is far more valuable than the 1/10 chance you nail the perfect mechanical wombo combo. It creates much more pressure, and also saves your mana - further extending how long you can stay in lane as well as increasing your pressure.
With a competent jungler, playing aggressive 100% of the time will almost certainly leave you dead and losing your tower. Look at it from Rek'Sai's (or any other jungler's) perspective: If Blitzcrank is going to pull me under tower and Jinx will follow up with damage, what is the likelyhood of survival? Slim to none. Now if Blitzcrank is in the middle of the lane throwing Q's left and right, what's the chance they can even kill you? Zero, without the assistance of the tower early game you'll almost guaranteed lose the 3v2 provided there weren't horrendous mistakes made.
This has gone on for MANY games for MANY years. If you played WoW, you'll certainly know the importance of saving cooldowns for important moments. A Rogue used Cloak of Shadows on his opener? Ok, he has nothing to stop magic now - dead Rogue. You can look all flashy and try to pull off the sickest combos you can think of, but in any competent player's mind all they need to do is counter what you've done. Same thing applies to League, albeit in much shorter cooldowns. There's a million examples I can give you for specific champion interactions, but in the long run it's your own knowledge of the game and cooldowns that will gain you the most benefit.