The boot order determines, which drive/source should be tried for booting and in which order. If there is no floppy disc drive installed, it does not make sense to include it in the order list as it wastes time on every boot.
On each boot, the BIOS goes through the boot order list until it finds a device/source which has some bootable data (Operating system) available. If the floppy disk drive is unavailable, the boot order skips to the next entry. If the floppy disc drive is available but either holds no disc or a disc without bootable data (boot sector), it skips to the next boot order entry, perhaps with an error message.
If your harddrive fails to boot or is not connected to your system, the BIOS again skips to the next boot order entry, such as a USB-Drive with bootable data or a CDROM or DVD drive which can hold a bootable medium (e.g. Windows installation CD).
The boot order does not prevent your PC from booting, but it can take unnecessarily longer to process, so it is recommended to put the harddrive or SSD with the operating system into the first slot of the boot order list.
EDIT: disable => prevent