At 13 this could either be a serious question or a provocation. You'll know best which it is.
Assuming she is sincere, I'd start by asking her what she thinks it is. Her answer may give you some idea of where to go next.
I'd explain that unless you are religious, and think of life as a sort of audition for heaven, there isn't really a 'point' as such, or a reason to live your life one way or the other. And I'd say that the fact we all die in the end either means nothing matters, or that everything matters enormously, and that either of those perspectives can be a blessing or a curse.
A more straightforward thing to say might be to think of herself not as one individual life, but part of a collective. The human species has been going for a good while now, and what certain people have done in their finite time has moved the needle in terms of where we are now - for better or worse. So maybe the point is to contribute in some meaningful and hopefully positive way to that ongoing project.
Or you can be a sentimentalist like me and say the point of life is to experience emotion - love, pleasure, awe, terror, grief and pain - and to NOTICE it all, to notice being human, because it's all we have and all there is.