Is it bad to integrate Firebase directly with the client (VueJs)?

I'm working on my capstone project and I've seen tutorials of people using Firebase by integrating it with VueJs and I did the same. My supervisor then told me that it was a very bad thing to do and that I should have a server side in my app that connects Firebase and Vue.

Is it because we don't want to put that code in the browser for everyone to see? Why are they so many tutorials doing it the "bad" way?