Yi is a very high powered champ who can outplay a lot of skill shots just by hitting Q and he's easy to play. A lot of people play him like this so when you throw their team off guard into saying, "yo why does yi have 3.5hp wtf" making him not only a tank but a tank with consistent dmg. If you just want to be a tank Stalkers Blade Cinderhulk will work wonders but you still have to output dmg and Yi excels in that always.

Things I feel are necessary in this build are the things I put in notes go and read them if you have certain questions