When and why does Git update update times on .keep packfiles?

Question

I am using Git version 2.4.11 in a Cygwin environment on a Windows 7 workstation for version control.

We make a copy of the local Git repository from time to time through Windows robocopy, which is sensitive to file modification times.

Some time ago, I created a file .keepfor my main packfile, arguing that incremental backups can skip my main batch file and therefore will be smaller and faster.

Instead, I find that the cookie modification time is constantly updated, as usual. Despite the fact that the file is the same length and contains the same bytes as before, this update to the modification time causes robocopyit to be copied again.

Under what circumstances touchgit packfiles .keep? Why is this done?