/etc/fstab defines the default configuration. It lists default filesystem/mount point/option combinations. When you mount a filesystem, if you don’t specify complete parameters and options,
mount will read the options from your
fstab. For example, you can type
mount will know where to put it, and you can type
mount will know where to find it. But if you want to be peculiar, you can type
mount /dev/sda1 /mnt/sdq17
mount will do that. Similarly, you can specify on the command line that a filesystem should be mounted read-only. Conversely, you can specify in
/etc/fstab that a filesystem should be mounted read-only by default, and then override that on the command line. And you can manually mount filesystems that aren’t in
/etc/fstab at all.
/etc/fstab identifies which filesystems are automatically mounted at boot time; they are mounted with their specified default options.
But also, just as you can type
mount /dev/sda1 /mnt/sda1, a program can execute
mount with a complete argument list, and then (in principle)
mount doesn’t need to access
/etc/fstab at all. And the operating system automagically knows where its root partition is, and automatically mounts the root filesystem very early in the boot process. In fact, if the OS needed
/etc/fstab to be accessible before it could do mounts, we’d have a chicken-and-egg problem, since
/etc isn’t accessible until the root filesystem has been mounted. The root partition is included in
/etc/fstab for the other reason that
/etc/fstab exists — to give
fsck a list of things to check.