Teacher diaries are used as development tools.
Example
After a class that went badly, the teacher makes notes in her diary about what happened, what she thought the causes were, ideas about how to change them and a short action plan.
In the classroom
Teacher diaries are a good way to start a course of development; a natural second step would be peer observation, i.e. getting another teacher to analyse the class in the same way.
Further links:
https://www.teachingenglish.org.uk/article/reflective-teaching-exploring-our-own-classroom-practice
https://www.teachingenglish.org.uk/blogs/grahamstanley/how-does-blogging-help-you-be-a-better-teacher-graham-stanley
https://www.teachingenglish.org.uk/article/jeremy-phillips-reflective-teaching-real-world