It's just a stand to hold equipment that is rack mountable, 19 inches wide with the propr mounting holes to screw it in.
Actually, I guess a "rack" is the stand with the equipment in it.
You can do without it until you have enough stuff lying around that needs to be mounted. You can use the equipment without it being rack mounted - it just makes it a lot more organized and easier to use.
The 'rack' holds the outboard gear... preamps, EQ, compressor, reverb, etc.. You do not need any outboard gear to get started. Mic, pre, soundcard, computer... good to go.