This is a good list by John Danaher. Read it in full here:
1. It helps to build the habit of writing:
2. It helps to generate writing flow states:
3. It helps you to really understand your area of research:
4. It allows you to systematically develop the elements of a research article
5. It enables you to acquire serendipitous research interests
6. It helps with networking and developing contacts
7. And yes, it also helps with teaching