There are many uses for machine learning in web development. I started out as a web developer and slowly drifted into recommendation engines and then onto neural networks and nlp.
It mostly would depend on your current focus, I would imagine. The book goes into detail on tools and approaches to build chatbots, for automating workflows for your users, for example. Tensorflow.js is now bringing the power of deep learning straight into the browser. So you can do near real-time classifications of text or images client-side with no need to ship data to a remote server. Thereby avoiding potential privacy concerns.
Today world seeks polyglot programmer and full stack. You should have knowledge on entire stack. Machine Learning comes behind backend stack. Output of machine learning can be shown in web for the clients. So it is better to learn it.