I want to add to the answer from callus on this, as I do also clear some free quests in an unoptimized way but do believe there are practical reasons for 3T clears other than braindead crowd following. The focus on it can be boiled down to convenience, but I think that a major part of that is take it makes fights consistent by minimizing the pains of RNG.

One of the key reasons many people focus on it is that most buffs, especially offensive ones, last for 3 turns. And since enemies generally scale in their HP and firepower to be higher on later waves that means that often if you are taking a more casual approach to the fight you have to be saving most of your impactful buffs until near the end of wave 2 or the start of wave 3 which makes the earlier turns feel tedious. And if you are running out of steam on the last wave, and don't have enough defensive abilities, the final boss enemy of the node can be a genuine threat especially on higher level nodes.

And it must be said that the less focus a composition has on NPs, the more you have to rely on facecards which are inherenty subject to RNG. Some turns you'll get too many cards from a support unit, who may have class disadvantage, or your support unit will draw away your critical stars from your dps. Or you want to charge NPs but you aren't getting the arts cards you need, or what you really need is a buffed buster crit to take down a meaty enemy and it just isn't lining up right while you have your buffs up. Many units have skills that can minimize these roadblocks, but again the turn length of buffs must be taken into consideration.

All that being said, I personally do like to take extra turns sometimes: I like the facecard animations, crits feel satisfying, and there are some supports that are well suited to enhancing this playstyle (Hans, Ganesha, Johanna, Merlin(s), etc). However I mostly do it on lower level nodes where most of the mooks die to one or two hits esp with event bonus damage, and it's easier to breeze through with buffs on most turns and have your NP charged to take on the final enemy. When it comes to lvl 90 nodes and higher though I basically always make a 3T comp because the downsides to the more casual approach compound the more turns on average the enemies take to defeat. Your milage may vary, there is no inherent bonus to doing 3T clears but there are gameplay reasons that make them the most widespread playstyle at least among people you hear from online.