She doesn't do "nothing" for team fights. Don't get me wrong, she's certainly not better than a lot of people.
But she's tanky (her cougar form = free tank stats) she can soak a lot of damage and make people chase her around.
She has a semi decent atkspd heal for your carry
Her Q in cougar form is excellent at last hitting champions. Can hit for around 700+ alone, + triforce proc and such.
I too, was once in your position. Wondering why people did it, until I tried it and I realized why.
So my advice? Try it.