^ what the title says
They're almost useless in team fights if they try not to die. If you're at a certain level, the enemy team will never position in such a way that would let the assassin kill the carries and run away.
So, how would an assassin help win a team fight or win the game in some other way?
Edit : I mostly play, for assassins: 
Sometimes: 
But I'm mainly talking about squishy champions like
The only assassins I really play are Rengar, Kha'Zix, and Akali, and all I do in teamfights is wait for the tanks to facecheck, wait for the hard cc to be used, and rush the adc. Late game, nearly any assassin can nuke an adc from 100%-0 in a few seconds
The Assassin is called an Assassin for a reason, they come out of nowhere and deal Critical Damage while a teamfight is ongoing. And if your being hunted down by the enemy team its best you stay with other players, especially if your not confident you can take on a horde of champions. Good luck friend.
First of all, you have to know when to play an Assassin. If the enemy team is mainly made of Fighters, your assassin won't live long enough to get much done. You should play assassins when the enemy team has 2-3 soft champions that do enough damage to be very threatening.
In the current style (heavy emphasis on Fighters), assassins are risky picks, and your team should be very careful about the choice. As it is, though, assassins are still really great to have on the team when the conditions are right. The best games I've played lately have been as assassins.
Again, the most important thing is making sure an assassin is the right choice for the team you have and the team you're playing against.
Opening the door for an assassin is the job of your team, you just have to be ready at the right moment. Even the best players will fall to assassins if the timing is right. Too many assassin players try to initiate fights, which is a mistake.
Coming from a Bronze assassin main + Zed main, you DON'T win as an assassin.
You create opportunities for your teammates to follow up on. If they can't win 4v4 you lose.
That's why I only worry about outplaying my enemies and always winning my lane and helping other lanes. The OUTCOME of the game doesn't matter to me. All that matters to me is killing and outplaying people.
You will have a similar outlook if you give it enough time.
When you have fun with Assassins, you need, need, NEED to know these things:
You must resign to the fact that you cannot carry.
You must resign to the fact that your team may not be able to maximize what you are able to do for them.
You must resign to the fact that you will be called out for things you've done nothing wrong because other people do not understand how assassins work.
