I mounted an external disk, looked at its contents in the file browser, and then did "safely remove drive". Surprise: a small window pops up stating that it's writing to the disk and warning me not to unplug the drive until it's done. What the heck is there to be written? I did not create any files, or change any file names, etc.
Every time you look at files or directories - access-times are written to the filesystem.
It a pretty big deal if you are doing forensics. In those cases you surly want to mount as read-only.
Also if access to the fs was previously cut in the middle of an operation (unmounted improperly) then the next time you mount it - a recovery will make modification to the fs to bring it to a consistent state by consulting the journal.