What "situational" items one should get during a game and why?
Starting items early game
Clarity - Buy this if you have skills you want to spam, like
Warlock's 
Fatal Bonds/
Shadow Word or 
Zeus's 
Arc Lightning. Don't buy this after early game.
Tango - Buy this if you're expecting to be harassed by autoattacks or DoT skills early game, for example: you're a melee hero who needs to get last hits and your enemies are ranged heroes, or heroes who can harass well like
Warlock. Buy Tangoes. You should usually get one stack of Tangoes anyway, but if you expect heavy harass get two stacks.