There is other equipment that you will be able to attach to the Spectrum.
The ZX Microdrive is a high speed mass storage device, and is much more flexible in the way it can be used than a cassette recorder. It will operate not only with SAVE, VERIFY, LOAD and MERGE, but also with PRINT, LIST, INPUT and INKEYS.
The network is used for connecting several Spectrums so that they can talk to each other - one of the uses of this is that you then need only one Microdrive to serve several computers.
The RS232 interface is a standard connection that allows you to link a Spectrum with keyboards, printers, computers and various other machines even if they were not designed specifically for the Spectrum.
These will use some extra keywords that are on the keyboard, but cannot be used without the extra attachments: they are OPEN#, CLOSE#, MOVE, ERASE, CAT and FORMAT.