Why do you have to sign in late at school

So I have free periods at the start of the day now as I've finished most of my GCSE subjects already so I come in late most days to the subjects I still study for but I hate having to sign the late book in the office as I'm sure the office staff wonder why I come in late like everyday and it's kind of awkward.

Why do you even have to sign in late? I asked and the school said it's so that the office staff can mark you in on the SIMS system as late, but say you come in at 11.15 and go to your timetabled class that you're meant to be in at that time, is the teacher not meant to change your absent mark to a late mark anyway so why do you have to sign in?