Why is the Putback initialized to '\n'?

Excuse me, I' m a freshman studying computer science. I'm trying to follow along this series and this is the first project I have tried. When I was studying the main.c in 01_Scanner, I can't understand why the Putback should be initialized to '\n'. I think about that if it could be initialized to 0 or just a whitespace. I think it doesn't matter.