When trying to add a new partition to a hard drive, you used to be able to just unmount the drive and perform your work. In SystemV init style systems, the drive would never just mount itself back up without any type of action being taken like the mount command being run. Contrast this to systemd, where if you just unmount a drive to perform a task like a fsck or a re-partition, when you try to access the device the drive is attached to for example, with a partitioning software like parted, systemd will mount the drive back up. This makes it very difficult to do something that used to be very simple. I seem to say that a lot with systemd.
To actually keep the drive unmounted the whole time you do your work, you have to perform the following steps.
During system boot systemd takes all of the fstab entries and generates native mount units for them. If you edit something in fstab you have to tell systemd so it can re-generage new updated units for the fstab entries. This is what the "systemctl daemon-reload" does. This will re-run all generators, and cause systemd to reload units from disk. Read more about it here.