1. When talking about what you are doing this weekend, this evening or in your free time, saying, "I am writing a novel," often wins.
2. You can ask inane hypothetical questions. If people know you are a writer, they assume it is for some book you are writing, even when it is not.
3. It gives you an excuse to read a lot. It is research. You have to study the craft.
4. You get to spend a lot of time in an imaginary world of your own making....and you call the time spent there productive.
5. Having the opportunity to meet other writers and if you are lucky, readers!