Right now ADC's are facing 2 main problems

1. They cannot kill tanks fast enough

2. They get one shot by everything, including said tanks

I believe tank varus is a solution to both of these problems.

For problem 1, varus has built in %max hp on his w, 5% per stack at max rank. This isn't a ton, but it makes a huge difference, and in the extended fights you create by building tank varus you can pop a ton of these stacks.

For problem 2, building tank makes you not get one shot (duh). If an assassin gets 8 kills at 10 minutes you are not surviving no matter what, but those types of games are the exception. Not super fed assassins will not be able to kill you in 1 rotation after you buy a single tank item, or even half of one.

If tank varus is so good, why does varus have a negative winrate?

Well, I think that's because people are building shitty items and maxing the wrong ability. People are building bork > guinsoos > terminus with berserkers greaves, but IMO bork is just not good on ranged anymore, and with bork being bad guinsoos has nothing to amplify and is trash too. I have been building kraken slayer > terminus > tank items with a defensive boot choice and it feels so good. In games where the enemy drafts a lot of ap you can go wits end third item and the extra attack speed is nice, while still giving you defensive stats. Runes I go are lethal tempo, triumph, legend: alacrity, last stand, conditioning, overgrowth with the standard atttack speed, adaptive force, flat hp bonus stats.

As for abilities, people are maxing q first, but I think maxing w first is way better. People have the false conception that varus is a poke champion so you should max the poke ability. You can play him that way and build lethality, but it's absolutely fucking useless unless you have the perfect game where every single enemy is extremely squishy. The way I play varus he is not a poke champion, he is a champion that goes all in and autos the enemies to death. Maxing w is the best way to support that plan.

context: common builds and ability orders taken from Ugg, my rank as of now is masters 80 lp. The game plays out completely differently in other ranks so maybe this doesn't work there, I don't know.