Sometimes, this question seems to be dogmatic. Like: If you don’t use your own material, you’re not a real teacher.
So let’s have a look at both possibilities, textbooks or own material.
Working with your own material
In most cases, “own material” is not as original as the expression suggests. When preparing their own material, teachers use texts from the internet, videos, pictures, excerpts and exercises from grammar books etc.
Authentic material taken from websites or newspapers for native speakers often has to be adapted,