Many reasons. Mainly to get feedback and reviews as mentioned by another user. I don't have a prominent blog or social media account, so getting users is quite difficult for me. I'm inspired by people like Al Sweigart and Allen Downey who provide excellent content on Python for free. I'd also like to open source my books sometime in future.

It's a common model recently to get word of mouth publicity for the book, to get some reviews to get the book off the ground.