Why does my manager want me to write down everything I do in a day?

This could be for many reasons. I wont assume why you may be singied out for example. But many work places require an account of the days work.

Working for a large mining company the paper work is thorough. As a supervisor I would do a hand over with the previous shift supervisor at about 5 am. We would discuss his shift, the department schedule and the mine schedule, safety issues, maintenance issues etc. Then I wiould setup my manning sheet for my crew, 45 employees. Do my shift start meeting and go through all that i mentioned previously and anything that had arisen after. Crew would go to work, I would attend a meerting with supervisors to duscuss the schedule and issues. So its still only 7:30am. I would have time to perhaps go through the training or development of crew members or deal with problems that had arisen..9am , 11am, 3pm were times when the crew members had to call me and tell me their truck loads or whatever it was they were doing, so it could be measured against the schedule. If tonnes were down, i would have tp maybe reallocate more equipment to that area. If there was any safety issues, incidents or breachss it would involve interviews, ffw testing, investigation, discipline etc..

I po 9⁹ crew members had to let me⁸880mm 0⁰00ùù⁸ì know if they we t ebmm⁸⁸⁸a were 15 mins behind or delayed and as a supervisor, i h⁹⁸ad to account for every hour. Fill out a shift report, write everything in my diary, as a legal protection .send emails to my superintendant and other requring consultation.

Basically they wanted to know the ins and outs of a ducks bum. But there was always information avaiil cablle to accounts ones 12 hour shuft.