When the answer to everything becomes 'hit harder', strategy is no more.
This can be a biased view, but for me, Tanks - the front line/vanguard - are the foundation of every PvP game with RPG aspects; let me explain why:
In a hypothetical team PvP RPG game, there's only one playable character, the Average Joe. He has avarage damage, avarage mobility, avarage range, etc; all Average Joe can do is deal damage and complete objectives. The point of the game is to complete objectives around the map - killing enemies is not an objective, but it does grants you advantages.
If two Average Joes fight, the one who was more advantages or the one who attacked first wins, which opens space to pursuit objectives, since one of the enemy Joes is dead.
It doesn't take too long before the players get bored of this hypothetical game, since there's only one character and there's only two things you can do: kill other Joes and complete objectives. It's then that a new character is introduced: Garen.
This one deals almost no damage, but he has so many control tools that none can pass through him; he is so tanky, only two or more Joes can take him down, but as counterpart, he moves slow. When Garen and Joe fight, it becomes a never ending battle: Garen will never be able to kill Joe, since he deals close to zero damage and Joe can't takedown Garen alone, since he's too tanky. Although Garen isn't capable of killing Joe, he can lock him down, preventing him from taking objectives.
For me, is in this exactly moment that 0 becomes 1: the need for strategy is born. 'Attack first' or 'Attack harder' no longer becomes a viable answer; now the players have to figure out the Garen vs Joe dilemma using creativity and smart thoughts: Joe can pursuit objectives that aren't guarded by Garen; one Joe can bait all Garens, while the other pursuit the objectives; two or more Joes can prepare an ambush to take down one Garen before the others appear; etc.
And that's just the start. When there's Joes and Garen in the same team, Garen can lock the enemy Joe down so the ally Joe guarantee a kill, follow by advantage; that's call teamwork, where two or more members of a team, with different skills, combine their different skills to archive something that they wouldn't be able to do alone. Is then that strategy becomes a thing, when there's a dilemma.
For me, League currently doesn't have true Tanks. As long as 
and others are allowed to have threatening damage while still being melted and dying almost as fast as a squishy, they'll never be true Tanks.
Champ 1 have 4000 HP, 200 Damage per second; Champ 2 have 2000 HP, [(200 + 5% enemy's Max HP) = 400] Damage per second.
Champ 1 is more tanky than Champ 2, but still, it doesn't make any difference. They would kill each other, because Champ 1, despite being tanky, has the same base damage than Champ 2 , and Champ 2 has HP scalling damage. In the end, it's just another Average Joe vs Average Joe 
Why Tanks need to have MaxHP% magic damage procs? 
Why damage is so accessible through runes and items? Why there's damage scaling with defensive stats? 
Why there's so much anti-tank stuff? 
Why half of the roaster can have viable access to 
?
If you take a Tank's tankiness, give him high base damage, he'll be just another Average Joe. And without a front line, without Tanks, there's no back line, there's no strategy.